fix(autogen): compare identity and computed defaults when compare_server_default is false

[Fire162]

Fixes #1877

Summary of Changes

When comparator plugins were reorganized, _user_compare_server_default() was registered with DispatchPriority.FIRST. If compare_server_default=False (or was omitted, taking the default value False), _user_compare_server_default() immediately returned PriorityDispatchResult.STOP.

Because STOP halts the entire comparator dispatch chain, later comparators (_compare_computed_default() and _compare_identity_default()) were never invoked, preventing autogenerate from detecting changes to Identity columns and emitting warnings for modified Computed columns.

This PR:

• Updates _user_compare_server_default() to return PriorityDispatchResult.CONTINUE when _user_compare_server_default is not a callable.
• Moves the check if migration_context._user_compare_server_default is False: return PriorityDispatchResult.STOP into _dialect_impl_compare_server_default(), so that plain server default comparisons are skipped while allowing computed and identity comparators to run.
• Adds regression unit tests in tests/test_autogen_diffs.py, tests/test_oracle.py, and alembic/testing/suite/test_autogen_identity.py.
• Adds an unreleased changelog note under docs/build/unreleased/1877.rst.

[sqla-tester]

Hi, and thanks for the pull request!

This project accepts pull requests only for issues that a maintainer has already marked with the open for pull requests label. That way we can settle on an approach before anyone spends time writing code.

This pull request references issue #1877, which hasn't been marked open for pull requests, so the change isn't authorized yet and I'm closing it automatically. Once a maintainer adds the label to #1877, reopen this pull request and it will stay open. If #1877 needs more detail before that can happen, a complete runnable example is usually the missing piece, and adding one to the issue is the fastest way to get there.

This is automatic and procedural. It isn't a judgment on your change, and nothing you've written here is lost.

Thanks for your interest in the project!