#### Description of the issue/feature this PR addresses:
In the Singapore localization (l10n_sg), reverse charge is modelled as a group tax pairing a -9% SRRC child with a +9% TXRC child, so the GST on a bill nets to zero while both legs are still reported in their respective GST return boxes. The child taxes "9% TXRC-TS" and "9% TXRC-ESS" shipped inactive, while their siblings "9% TXRC-N33" and "9% TXRC-RE" shipped active. Because children_tax_ids is a many2many onto account.tax (which has an active field), inactive children are filtered out of the group, so the groups "Reverse Charge - SRRC + TXRC-TS" and "Reverse Charge - SRRC + TXRC-ESS" only kept the -9% SRRC leg and computed a wrong GST amount, while leaving the +9% leg out of the GST return.

#### Current behavior before PR:
A vendor bill of S$10,000 taxed with "Reverse Charge - SRRC + TXRC-ESS" (or "+ TXRC-TS") shows 9% GST = -S$900.00 and a total of S$9,100.00 instead of net S$0.00 / S$10,000.00. The +9% TXRC leg never reaches Box 5 / Box 7 of the GST return. The sibling groups "+ TXRC-N33" and "+ TXRC-RE" are unaffected because their children are active. The only workaround is to manually activate the two child taxes.

#### Desired behavior after PR is merged:
The "9% TXRC-TS" and "9% TXRC-ESS" child taxes are active by default, so the group taxes aggregate both legs: a S$10,000 bill shows 9% GST = S$0.00 with a total of S$10,000.00, and both reverse charge legs land in their GST return boxes. New SG databases get this from the tax template; existing SG databases get the two taxes reactivated by a migration on upgrade.

opw-6199248

---
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Forward-Port-Of: odoo/odoo#267670

**Steps to reproduce on a new db:**
(bug also reproducable on runbot but the impact 
is less easy to compute because of influence of other
existing companies)

- create a new company as company 2 and use the existing 
default company as company 1.
- create a warehouse for both company
- for both comp, in settings for the 'fiscal localization' set 
Package : Generic Chart of account, if not already set
(to have account journals).
- for both comp, in settings for inventory valuation set 
'periodic' and for periodic valuation set 'daily'

From company 1 : 
- create a storable product with standard price method and 
set a cost of 30
- set an onhand quantity of 1
if you navigate to 'inventory valuation' you'll see that : 
  - initial balance is 0
  - ending stock is 30 
  - the variation lines have a balance of 30
  - all of this is expected

From company 2 :
- change the cost of the product to 10
- set an onhand quantity of 1
if you navigate to 'inventory valuation' you'll see that : 
  - initial balance is 0
  - ending stock is 10 
  - the variation lines have a balance of 10
  - all of this is expected
 
From any company :
- navigate to 'scheduled actions' and select the action 
'Stock Account: Inventory Valuation Closing'
- click on 'Run Manually'
- navigate to 'inventory valuation'

**Current behavior:**
with company 1 selected :
  - the initial balance is now 30 
  - ending stock still 30
  - no variation lines
  - the initial balance was correctly increased by the closing entry

with company 2 selected: 
  - the initial balance is now 40
  - the ending stock is still 10
  - the variation lines credit 30 in stock valuation

In company 2 the closing entry debitted 40 in stock valuation 
instead of 10 which increased the initial balance to 40 instead of 10

If you open the journal items you'll find the closing amls have a 
balance of 40 instead of 10

**Cause of the issue:**
The _cron_post_stock_valuation() method 
calls action_close_stock_valuation() on both companies
https://github.com/odoo/odoo/blob/bfa39854e56da4bf23295d62f63d66973ad0d78e/addons/stock_account/models/res_company.py#L143-L144

This methods calls _action_close_stock_valuation
with a context modified with only self.env.company.ids
in 'allowed_company_ids'
https://github.com/odoo/odoo/blob/bfa39854e56da4bf23295d62f63d66973ad0d78e/addons/stock_account/models/res_company.py#L56

This is needed because inside stock_value() we use 
the total value of the product 
https://github.com/odoo/odoo/blob/bfa39854e56da4bf23295d62f63d66973ad0d78e/addons/stock_account/models/res_company.py#L92
which will be the sum of the values of the product 
for each company inside allowed_company_id
https://github.com/odoo/odoo/blob/bfa39854e56da4bf23295d62f63d66973ad0d78e/addons/stock_account/models/product.py#L274
So in case action_close_stock_valuation() was called from 
the 'generate entry' button from the inventory valuation view
we need only the main company selected to be in the
'allowed_company_ids' so that the inventory value is computed 
based only on this company (as is the accounting value).

The problem is that this does not work when calling the 
method from _cron_post_stock_valuation because then
there is no 'allowed_company_ids' in the context (because it was
called from _process_job() with a new env).
so self.env.company will be the company of the user 
which will be company 1.
https://github.com/odoo/odoo/blob/bfa39854e56da4bf23295d62f63d66973ad0d78e/odoo/orm/environments.py#L243

Therefore when _action_close_stock_valuation will be 
called on company 2, in the context, allowed_company_ids 
will be company 1. Then, when computing 'products', with_company() 
will add self (company 2) to the context.
https://github.com/odoo/odoo/blob/616e82d7b3a53b1facf481e783baed3e99393d3c/addons/stock_account/models/res_company.py#L151-L152
So stock_value will return the sum of the total_value 
of each product for company 1 and company 2 which is 40 (instead 
of 10 for just company 2)
https://github.com/odoo/odoo/blob/616e82d7b3a53b1facf481e783baed3e99393d3c/addons/stock_account/models/res_company.py#L242

We then create the closing accounting entry to match the 
accounting value with the stock value, which explains why 
the new initial accounting balance of company 2 is 40.

**fix:**
We set the context using self instead of self.env.companies
This makes more sense as both in the cron use case and 
the generate entry use case the stock value we want is the 
one of the company in self. 
- In cron use case, it's obvious as the method is called 
in a for loop on each company
- In the generate entry use case, self will also be the main 
company, because it's called, in actionGenerateEntry, 
on this.companyId 
https://github.com/odoo/odoo/blob/616e82d7b3a53b1facf481e783baed3e99393d3c/addons/stock_account/static/src/stock_valuation/controller.js#L75
which is computed based on the get_report_values
https://github.com/odoo/odoo/blob/616e82d7b3a53b1facf481e783baed3e99393d3c/addons/stock_account/static/src/stock_valuation/controller.js#L21
https://github.com/odoo/odoo/blob/616e82d7b3a53b1facf481e783baed3e99393d3c/addons/stock_account/static/src/stock_valuation/controller.js#L28-L30
Which returns the main company
https://github.com/odoo/odoo/blob/616e82d7b3a53b1facf481e783baed3e99393d3c/addons/stock_account/report/stock_valuation_report.py#L29


Most importantly, this is also aligned with how the accounting 
values are computed.
https://github.com/odoo/odoo/blob/616e82d7b3a53b1facf481e783baed3e99393d3c/addons/stock_account/models/res_company.py#L103-L105


opw-6237402

Forward-Port-Of: odoo/odoo#270127
Forward-Port-Of: odoo/odoo#266932