Closes#24090
Enhances the existing `TestFinalizeStaleChatDebugRows` test with three
missing coverage areas:
1. **Error JSON preservation**: verifies pre-existing error payloads are
not overwritten by finalization
2. **Timestamp correctness**: verifies `updated_at` and `finished_at`
match the `@now` parameter across all finalized row paths
3. **Null error preservation**: verifies finalized steps that had no
error keep a null error column
No production code changed. Test passes against Postgres.
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<details><summary>Review notes</summary>
- Enhances existing test rather than adding a new one, the existing test
was the right place
- Covers stale, orphaned, and cascade finalization timestamp assertions
- Preserves both pre-existing error JSON and null error values during
finalization
</details>
Stephen Kirby
03c5ae3f70