This pull request introduces improvements to the handling of Chilean electronic document sequences, enforces stricter validation for certificate data, and modernizes string formatting throughout the codebase. The most significant changes focus on ensuring that document folio numbers are correctly managed in accordance with available CAFs, making the certificate model more robust, and updating code style for clarity and consistency.

This improvement is mostly made because CAFs have expiration dates and fixed folio numbers. If the expiration date is reached but the folios were not fully consumed, the system will try to generate a document with following number in the sequence but an error will be raised because that number belongs to an expired CAF, althought the user had already uploaded a new CAF.
To fix this, whenever the system looks for the highest number to set to a document, it previously check if that number is between the folio numbers of a valid CAF file.

### Chilean electronic document sequence management

* Added `_get_last_sequence` method to `account_move.py` to ensure that the folio number used for Chilean electronic documents (DTE) is valid and corresponds to an available CAF. If no CAF is available for the current folio, the sequence is reset using the document type's prefix and starting number.

### Certificate model validation

* Made `subject_serial_number` a required field in the `Certificate` model, enforcing that every certificate has a valid owner document and improving data integrity. We made this change to prevent the field being empty after loading the certificate in Odoo, since it is required by the SII to validate invoices.

Forward-Port-Of: odoo/enterprise#92208