[16.0, 17.0, 18.0] mail_template_multicompany. Mail templates are not accessible across multiple companies in multi-company environments

[stferraro]

📝 Description In a multi-company Odoo environment, mail templates (mail.template) assigned to a specific company are not accessible when switching to another company. This issue prevents users from sending emails using templates that belong to a different company, even if they have access to both companies.

Steps to Reproduce

1. Go to Settings > Technical > Email > Templates.
2. Create a new mail template and set company_id to Company A.
3. Switch to Company B (User has access to both Company A and Company B).
4. Try to use the template in a related model (e.g., Sale Order or Invoice).

Expected Behavior:

The user should be able to access and use the template regardless of the assigned company.