Issue:
In a multi-company environment, when an attachment is uploaded in the chatter, the resulting `ir.attachment` record incorrectly sets the company_id to the current user's default company. This occurs regardless of the active company or the company associated with the record the attachment is linked to.

Steps to reproduce:
In a multi-company setup, activate only Company B. Open any chatter and attach a file.
Navigate to Settings > Technical > Data Structure > Attachments.

Current behaviour:
The created attachment has its company_id set to the user's default company, rather than the currently active company, or the company of the record, even when the record has a company_id field. This behavior introduces inconsistencies in data visibility, particularly when attachments appear to belong to a company different from the one associated with the related business record.

Behaviour After Fix:
If the related record (i.e., the model the attachment is linked to) contains a company_id field, its value will be used as the attachment's company_id otherwise, the attachment's company_id will be set to the currently active company's id.

This logic ensures proper alignment between attachments and their related business records and also maintaining consistency in multi-company scenarios.

task-4563173


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Forward-Port-Of: odoo/odoo#277582
Forward-Port-Of: odoo/odoo#235665

Issue:
When a shopper pays for an online order entirely using a reward (e.g. a discount code covering 100% of the total), the order total is zero. With automatic invoicing enabled, an invoice with an amount of 0 is created. But, the 0 invoice is never emailed to the customer. Whereas, for orders where the total is more than 0, an invoice is created, then emailed to the customer. The customer should be emailed the invoice, even if its amount is 0.

Steps to reproduce:
1. Enable automatic invoicing.
2. Create a code for a 100% discount.
3. As a shopper, add a product to cart on the website.
4. While checking out, apply the 100% discount code to the order.
5. Complete the checkout.
6. Confirm that an invoice was created and posted, but was not emailed to the customer.

Explanation:
Normally, order confirmation and invoicing are handled by the `_post_process` method on the `payment.transaction` model. With automatic invoicing enabled, `_post_process` confirms the sale order, creates the invoice, and sends the invoice via `_send_invoice` (another method on the `payment.transaction` model). If `sale.async_emails` is enabled, `_post_process` will trigger a cron that invokes `_send_invoice` instead of invoking it directly.

When an order is fully covered by a reward, there's nothing to pay. In this case, no payment.transaction record is ever created, and `_post_process` never runs. Instead, the order is confirmed through the `_validate_order` method on the `sale.order` model. The `sale_loyalty` module extends `_validate_order` so that, with automatic invoicing enabled, it will create and post an invoice for zero-amount orders. But, nothing in this path ever calls `_send_invoice` or an equivalent. So, the invoice is created and posted but never sent.

Solution:
This adds logic for sending invoices to the extension of `_validate_order` in the `sale_loyalty` module. We mirror the logic used in `_send_invoice` in the `payment.transaction` model.

Notes:
There is duplicated code from `_send_invoice` in this fix. That is because `_send_invoice`, a method on the `payment.transaction` model, can't be used in this flow. A fix that avoids code duplication would require serious refactoring.

This will never trigger a cron to send the invoice, even if `sale.async_emails` is enabled. That is because the cron invokes `_send_invoice`. Since fully reward-covered orders are probably not common, any performance benefits of using a cron are probably not significant. But, making a new cron to be used in this case is also an option.

opw-6363334

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Forward-Port-Of: odoo/odoo#275190

_**Description of the issue/feature this PR addresses:**_
SaleOrder._onchange_order_line retrieves the linked lines of each order line by calling SaleOrderLine._get_linked_lines(). 
That method filters the whole order_line recordset on every call.
Since the onchange is triggered by any change to order_line, this noticeably slows down every modification of the lines on large orders even when the order contains no combo product at all.

_**Current behavior before PR:**_
For every order line, _onchange_order_line calls line._get_linked_lines(), and each call re-scans the entire order_line recordset with filtered(). The cost therefore grows with the total number of lines (regular lines included), so editing any line on a large order is sluggish.

_**Desired behavior after PR is merged:**_
The {line: linked_lines} mapping is computed in a single pass via the new method SaleOrderLine._get_linked_lines_by_line(), called once per onchange instead of once per line. 
The method _get_linked_lines() is kept as a thin single-record wrapper delegating to the batched method, preserving backward compatibility for external callers. Behavior is unchanged and only the performance is improved: the onchange now scales linearly with the number of order lines.

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Forward-Port-Of: odoo/odoo#277185