Email attachment logic for electronic invoicing has been moved from shared mail components into the relevant accounting models. This makes the system easier to maintain and reduces the risk of errors when sending localized accounting documents by email.

Purpose of this task is to cleanup attachment management done in composer overrides
in account modules and move it to model.

See community PR for more details.

Task-2710804 (Mail: Clean MailThread API)
Task-2792146 (Mail: Move model-dependent code from composer / template)

Test support code for document-related spreadsheet behavior was moved to the module where it belongs. This is an internal cleanup that helps keep future development and testing better organized, with no expected impact on daily users.

mocked methods for `documents.document` belongs to `documents_spreadsheet` module.

A new shared module centralizes the common business logic used by Odoo's document extraction features. This should make invoice, expense, purchase, and recruitment extraction easier to maintain and more consistent, with no major direct change for end users.

This commit adds a module that contains the common business logic of all *_extract modules.