[FerDeNetlab]

Fixes for OCA#6046 found while running a full 19.0 → 20.0 migration (crm, sale_management, purchase, stock, account + l10n_mx, project, hr, mrp, point_of_sale; demo data, 131 modules):

• Keep attachments of removed models. v20 added IrModel._unlink_related_attachments (odoo/addons/base/models/ir_model.py), an @api.ondelete(at_uninstall=False) that deletes all ir.attachment rows of the model (including attachment=True binary fields) and marks their files for GC. In v20 those methods are only skipped for modules in registry.uninstalling_modules, which is still empty when _process_end runs, so every obsolete ir.model unlinked there loses its attachments while _drop_table keeps the table. The patch replaces it with a logging no-op, in line with "never drop tables".
• Validate views one by one. The ValidationError handler of _check_xml reads self.xml_id/self.model, but _validate_module_views and create() call it on multi-record recordsets → Expected singleton aborts the upgrade instead of logging a warning.
• Catch follow-up errors of a swallowed _raise_view_error. As it doesn't raise anymore, validation goes on and fails with AttributeError/KeyError/NameError/ValueError (e.g. _check_field_paths, _validate_domain_identifiers), which nobody caught.
• Use cr.savepoint() in unlink. str(uuid4) never called uuid4, so all savepoints shared one name and were never released; the cursor helper also restores the ORM state on rollback.
• Respect country_ids when auto installing in update_list, like button_install does (otherwise e.g. account_peppol gets installed on databases without a Peppol country).

With these (and migration scripts for the modules above) the test database migrates with all 1692 views valid and accounting data identical to v19.

Assisted-by: Claude Opus 5.5 (drafted with AI assistance, reviewed by me)