feat: store hash of dag_version.version_data to avoid loading/comparing large manifests

[anmolxlight]

Summary

Store a hash of dag_version.version_data to avoid loading and comparing the full JSON manifest on every DAG parse.

Problem

SerializedDagModel.write_dag's "serialized hash unchanged" fast path refreshes DagVersion.bundle_version / version_data in place, comparing the full stored version_data against the incoming value:

1. _prefetch_dag_write_metadata loads the full DagVersion row — including the entire version_data JSON — for every DAG in the bulk write.
2. The steady-state same-bundle case re-compares the full version_data dict each parse.

Solution

Persist a version_data_hash (md5 of canonical JSON, String(32), nullable) on dag_version and compare/prefetch that instead of the full blob:

• DagVersion model: new version_data_hash column + compute_version_data_hash() static method
• _prefetch_dag_write_metadata: uses load_only() to skip loading the version_data JSON column entirely
• Fast path comparison: compares version_data_hash instead of full dicts
• In-place refresh: updates version_data_hash when bundle metadata changes
• New DagVersion rows: computed on creation

Verification

• All 66 test_serialized_dag tests pass
• All 8 test_dag_version tests pass
• All migrations chain correctly from latest 9ff64e1c35d3

Closes: #68567

[github-actions]

uv.lock on main just moved via #68710 ("Add Ray constraints for Python 3.14 in Google provider"), commit 9c49080 and this PR currently conflicts.

Quickest fix:

git fetch upstream main && git rebase upstream/main
rm uv.lock && uv lock
git add uv.lock && git rebase --continue
git push --force-with-lease

Automated nudge — ignore if you're not ready to rebase. This comment is updated in place on future uv.lock bumps.