When posting an invoice, the system creates:
- Journal Entry: Dr 131 (Receivable) / Cr 511

Then the system creates a deferral entry:
- Deferral entry: Dr 511 / Cr 3387 (Payable)

Falsifying the Balance sheet report, in the accounts receivable and accounts payable indicators

The issue was that account 3387 was configured as `Payable`, which caused the system to generate both Receivable (131) and Payable (3387) for the same partner. This is incorrect because account 3387 represents "Unearned Revenue", which is a  current liability, not a payable account.

By changing the account type from `Payable` to `Current Liabilities`,
the deferral entry now correctly reflects that 3387 is a current liability account, preventing the incorrect reconciliation behavior where both receivable and payable entries were created for the same partner.

After this fix:
- Entry: Dr 131 (Receivable) / Cr 511
- Deferral: Dr 511 / Cr 3387 (Current Liabilities)

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:




---
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#### [FIX] snailmail: extract report PDF generation function
We extract a function `_generate_report_pdf` from `_fetch_attachment`
to create the report PDF (and its filename).
The resulting PDF's margins are fixed and a cover page is added to it
after the function is called in `_fetch_attachment`.

The new function is extended in the related enterprise commit to generate the
followup report inside `_fetch_attachment` (when sent via snailmail).
This way it will respect the cover page option and page layout / size requirements.
(See the related enterprise PR for more details.)

#### [FIX] snailmail: extract letter resending function

We extract a function `_resend_letters` from the `update_resend_action`.
It handles the regeneration of letters after the cover option has been updated.
This way the resending logic can easily extended to adjust the logic depending
on attributes of the letter.

The new function is extended in the related enterprise commit to disable
the resending for followup report letters.
This is necessary because the followup report requires special options
to be generated that are not available at the point of the regeneration.

#### references

opw-5160121
opw-5209504
opw-5226366

#### [FIX] snailmail_account_followup: fix address, cover page and layout

Currently there is the following potential problem when sending
the followup report via snailmail.
1. The address generation is not adjusted for snailmail. That can
   lead to problems with the service we use to send the actual letter.
   They validate the address rather strictly.
2. The cover page option does not work; it does not add a cover page.
   So we can not work around problems with the address generation
   by adding a cover page.
3. The layout / dimensions / margins of the generated document / PDF may not work
   with our current snailmail provider (Pingen). But there is no error
   message about it. (Although we do have something in the usual
   snailmail flow)
4. In case the address is invalid we do not try to "print" / send the letter,
   so the user does not receive any feedback.
   This could be an issue in case multiple follow-up reports are sent
   at the same time.

This commit fixes these issues. (See below for details.)

(1)
The logic for this already exists but it is only activated when
a context key is set. This is not the case currently.
After this commit we do set the key.

(2) & (3)
The issue is that we generate the PDF attachment before creating the
'snailmail.letter' record.
In the usual snailmail flow the PDF attachment generation is handled during the sending and
printing (in function `_fetch_attachment` on model 'snailmail.letter').
There is some special logic to
- add a cover page to the report PDF (if the option is selected)
- make sure the page dimensions of the PDF are okay
- overwrite the margins of the PDF with white to make sure the PDF is
  not rejected by Pingen because of this

But all this only happens if we do not have an attachment already.
(So it does not happen currently with the followup report)
For this a function called `_generate_report_pdf` was extracted from `_fetch_attachment`
in the related community commit to generate the report PDF (and its
filename). The function is extended here to be able to generate the
followup report.

(4)
We try to print / send the letter even if the address is invalid

Reproduce (i.e. for the cover page issue; but it explains how to get
the PDF that will be sent in general)

1. Install `snailmail_account_followup`
2. Create an overdue invoice
3. Set the "Add a Cover Page" option
   (Settings -> Accounting -> section "Customer Invoices")
   - enabled to test for the cover page
   - disabled to test that the address generation is adjusted
4. Send a follow-up report:
   - On 17.0: Accounting -> menu: "Customers" /  "Follow-up Reports"
     -> click on a line / partner -> button "Follow up"
   - On 18.0+: partner form view -> tab "Accounting"
     -> section "invoice follow-ups" -> button "Send"
5. Go to the snailmail letter:
   In debug mode: Settings -> menu: "Technical" -> section: "Email" -> "Snailmail Letters"
   (or just search for "snailmail" in the main screen)
   And select the letter
6. Download the PDF document

#### [FIX] snailmail_account_followup: forbid regenerating failed letters

The wizard to resend failed letters which allows to change the
cover page option is broken: The follow-up report can not be regenerated
correctly because it requires special follow-up specific `options` that are
lost after the initial pdf generation for the letter.

Currently it can happen that the follow-up PDF is regenerated but
without (actual) content (table listing the overdue amounts).

After this commit we cancel the snailmail letters and show an
error notification indicating that the followup needs to be done again to
create a new letter.

Reproduce
(needs credit on IAP or locally edit this function https://github.com/odoo/odoo/blob/3ffd51f1cb18e3f4fb0367c4a498d7438e0c0357/addons/snailmail/static/src/core_ui/message_patch.js#L11
to open the resend wizard `this.openFormatLetterAction()` for `sn_credit` error or always)

1. Install `snailmail_account_followup`
2. Create an overdue invoice
3. Ensure the address of the partner causes issues with Pingen
4. Ensure the cover page option is disabled:
   Settings -> Accounting -> section "Customer Invoices"
5. Send a follow-up report:
   - On 17.0: Accounting -> menu: "Customers" /  "Follow-up Reports"
     -> click on a line / partner -> button "Follow up"
   - On 18.0+: partner form view -> tab "Accounting"
     -> section "invoice follow-ups" -> button "Send"
6. Make some modifications like editing the follow-up message or a custom attachment
7. Download the snailmail letter PDF (see previous commit for details)
8. In the chatter go to the message saying "Letter sent by post with Snailmai"
9. Click on the red symbol (paper plane) next to the name
10. A "Format Error" wizard should show up
11. Select "Add a Cover Page"
12. Click the button "Update Config and Re-Send"
13. Download the snailmail letter PDF (see previous commit for details)
14. Compare PDFs from 7 and 13; they are different (not just the cover page)

#### references

opw-5160121
opw-5209504
opw-5226366

Steps to reproduce
-----
- Enable batch pickings
- Create a product
- Create 2 receptions for the product (qty > 1)
- Create a batch with the 2 transfers
- Open the batch in barcode
- Scan part of both pickings
- Go back to the barcode main screen
- Open the batch again

> Both pickings have their demand = partially delivered quantity

Cause
-----
When leaving the page, we trigger

https://github.com/odoo/enterprise/blob/91d6a096e88e4f11d7504d7a4052a57e2cb09ca8/stock_barcode/models/stock_move.py#L65-L68

in which we end up merging the moves together

https://github.com/odoo/enterprise/blob/91d6a096e88e4f11d7504d7a4052a57e2cb09ca8/stock_barcode/models/stock_move.py#L51

This has been added by 9753c24 (ade0bef in 17.0)

The problem is that `_merge_moves` merges all of the moves into the first of `merge_into`

https://github.com/odoo/odoo/blob/26761e04bb648b46cd35697c6cbc8ed1e27fef90/addons/stock/models/stock_move.py#L1086-L1088

This, however, doesn't make much sense for batches because the moves can be from different pickings.

-----
Ticket:
opw-5163740