Convert flexible field values to SQL before storing them

[Maxr1998]

Description

This is required to support multivalued fields as flexible fields.
See #5698 for more details.

To Do

• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

(will add those later)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Copilot]

Pull Request Overview

This PR adds conversion of flexible field values to their SQL representation before inserting them into the database, enabling support for multivalued flexible fields.

• Applies to_sql on each flexible field value prior to storage.
• Removes keys from the dirty set as before, then converts and inserts the SQL value.
• (To be added) Changelog and tests for the new behavior.

Comments suppressed due to low confidence (2)

beets/dbcore/db.py:607

• [nitpick] Overwriting the original value variable may reduce readability; consider assigning the converted value to a new variable (e.g., sql_value) and using that in the mutate call.

                    value = self._type(key).to_sql(value)

beets/dbcore/db.py:607

• Add tests to cover the new to_sql conversion path for flexible fields, verifying that multivalued and edge-case values produce the expected SQL representations.

                    value = self._type(key).to_sql(value)