Store a hash of dag_version.version_data to avoid loading/comparing large manifests on parse

### Description

`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:

```python
# airflow-core/src/airflow/models/serialized_dag.py
bundle_metadata_changed = (
dag_version.bundle_version != bundle_version or dag_version.version_data != version_data
)
```

`version_data` is a free-form JSON column (e.g. an S3/custom-bundle manifest). When it is large, two things get expensive on every parse:

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 (only skipped when `bundle_version` already differs, thanks to `or` short-circuiting).

Proposal: persist a `version_data_hash` (e.g. md5 of the canonical JSON) on `dag_version` and compare/prefetch that instead of the full blob. The prefetch then loads only the small hash, and the change check compares hashes.

### Use case/motivation

Keep DB-side parsing cheap and memory-flat as `version_data` grows (large manifests from S3/custom bundles). Today the built-in bundles keep `version_data` small or empty (GitDagBundle doesn't set it), so this is a forward-looking optimization rather than a current hotspot — surfaced in review of #68336.

### Related issues

Follow-up from review on #68336 (review comment by @uranusjr). The in-place refresh logic was introduced there.

### Are you willing to submit a PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

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Drafted-by: Claude Code (Opus 4.8); reviewed by @ephraimbuddy before posting