Preserve max_length and decimal constraints in alter_column replay

[ebrake]

This PR preserves constraint-related alter_column changes during migration generation and replay.

Specifically, it keeps these fields from being dropped when migrations are generated or replayed:

• max_length
• max_digits
• decimal_places

It also adds focused regression tests for those cases.

Problem

makemigrations could repeatedly generate the same alter_column(...) operations for constraint-only field changes because those constraint fields were not preserved
through the Python migration round-trip.

That meant Oxyde could detect the change, write the migration, and then lose part of that change when replaying migrations to rebuild schema state.

Changes

• preserve max_length, max_digits, and decimal_places in migration generation
• preserve the same fields in migration replay
• preserve the same fields when building new_field for alter_column
• add targeted tests in test_migrations_detection.py

Scope

This fixes constraint-field round-tripping for alter_column.

It does not attempt to fix other unrelated repeated alter_column diffs.

Testing

oxyde/tests/unit/test_migrations_detection.py

[ebrake]

@mr-fatalyst I just patched the ones I was running into, which are common, but there are others! https://pydantic.dev/docs/validation/1.10/usage/types/

[mr-fatalyst]

I recently added this fields and didn't test everything yet. Thanks for PR!