compare identity, computed defaults when compare_server_default is false

[Fire162]

Fixes #1877

Problem

In 1.18.0, column comparators were moved to PriorityDispatcher. _user_compare_server_default was registered at DispatchPriority.FIRST under the "server_default" subgroup. When compare_server_default=False (the default setting in Alembic migration contexts), _user_compare_server_default returns PriorityDispatchResult.STOP. This aborts all subsequent comparators registered in the "server_default" subgroup before _compare_computed_default or _compare_identity_default can run.

Additionally, _setup_autoincrement was registered under the "server_default" subgroup, causing autoincrement metadata on altered columns to be dropped whenever server default comparison was stopped.

Solution

1. Register _compare_computed_default and _compare_identity_default at DispatchPriority.FIRST in the "server_default" subgroup so they execute before _user_compare_server_default (registered at DispatchPriority.MEDIUM).
2. Set alter_column_op.existing_server_default = conn_col_default in both _compare_computed_default and _compare_identity_default.
3. Return PriorityDispatchResult.STOP in _compare_identity_default whenever an identity default is present, preventing fallback into plain server default comparison.
4. Register _setup_autoincrement under the "autoincrement" element so it runs independently of server default comparison.
5. Add unit tests in CompareServerDefaultTest verifying that computed warnings, identity modifications, and autoincrement metadata flags are preserved when compare_server_default=False.

[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!