[14.0][FIX] partner_multi_company: sync user-partner companies on install

[Mert-coderoid]

The post_init_hook only copies the legacy partner.company_id (Many2one) into the new company_ids (Many2many) field, ignoring res.users.company_ids.

For users allowed in more than one company, this leaves their own partner record visible in only the company that happened to be in the legacy partner.company_id. Logging in under any other allowed company fails with AccessError when _login reads user.tz (related field on partner). The unhandled AccessError surfaces as a 500 page since the navbar template's fallback also goes through the partner record.

Reproduction (clean 14.0)

• Two companies: A, B
• User U: company_id=A, company_ids=[A, B]
• Install partner_multi_company: post_init_hook sets U.partner_id.company_ids = {A}
• Log in as U, switch to company B → 500

Fix

Add an extra INSERT in post_init_hook that adds, for every user with a partner_id, every company the user is allowed in. The same SQL is run as a 14.0.1.2.0 post-migration so existing installations get the backfill on upgrade.

Two new tests cover create-time and write-time sync (handled by the existing res_users.py override, included for regression safety).

Closes #995
Closes #438

[OCA-git-bot]

Hi @renda-dev, @PicchiSeba, @aleuffre,
some modules you are maintaining are being modified, check this out!