test(db): add migration diff and MySQL generator golden tests

[GaspardKirira]

Summary

Add automated tests for the schema diff engine and MySQL migration SQL generator.

Generated migration operations and SQL must be deterministic and reversible.

Current behavior

The diff engine currently supports:

• table creation;
• table removal;
• column addition;
• column removal;
• index creation;
• index removal.

The MySQL generator produces up and down SQL scripts for these operations.

No automated tests currently protect the generated output.

Expected behavior

Tests must compare migration operations and generated SQL against reviewed expected output.

Scope

• Test an empty diff.
• Test table creation.
• Test table removal.
• Test column addition.
• Test column removal.
• Test index creation.
• Test index removal.
• Test multiple operations in one diff.
• Test MySQL type generation.
• Test identifier quoting.
• Test nullable and non-null columns.
• Test primary keys.
• Test auto-increment columns.
• Test unique columns.
• Test default values.
• Test generated down migrations.
• Preserve the existing public DB API.

Suggested files

• modules/db/tests/migration_diff_test.cpp
• modules/db/tests/mysql_generator_test.cpp
• modules/db/tests/golden/...

Acceptance criteria

• Every currently supported operation has test coverage.
• Generated up SQL is compared against expected output.
• Generated down SQL is compared against expected output.
• Down operations are emitted in reverse order.
• Test expectations are readable and easy to review.
• Repeated runs produce identical output.
• Existing public APIs remain unchanged.

Non-goals

This issue does not include:

• altered-column detection;
• renamed-column detection;
• SQLite SQL generation;
• PostgreSQL SQL generation;
• foreign-key generation.

Suggested labels

• scope:db
• type:ci
• tests
• help wanted
• status:ready