Fix response finalizer JSON validation failure

[Romonaga]

Change Type

• Feature
• Bug fix
• Refactor
• Documentation / workflow artifact
• Repo organization / cleanup
• Other: ___

Linked Workstream Change

• Linked Verlyn change: circuit-shelf-33
• No change applies - reason: ___

Verlyn PR Metadata

Verlyn PR creator: ___

Acceptance Criteria

• Malformed or non-JSON response-finalizer output no longer produces an opaque 'Response finalizer did not return valid JSON' validation result for Ask responses.
• The validation result remains explicit about source-grounding issues without discarding a usable answer when retrieved material exists.
• Regression coverage exercises non-JSON finalizer output and any supported JSON-extraction fallback.
• Targeted backend tests pass; frontend build is run only if the API/UI contract changes.

Verification Runs Completed

• Session baseline
• Architecture check (if configured)
• Workstream validation: python scripts/repo_helpers/workflow/validate.py <id> --strict
• Other:
  • ___

Coverage Summary

• 7 of 8 tracked work items are complete.

AI Assistance Disclosure

• AI-assisted - tool(s) used: Codex
• Human-authored only

Accountable human owner: hellweek

Implemented with Codex AI assistance in the governed repo workflow. AI assistance was used to inspect the response-finalizer parser, make the focused backend change, add regression tests, run targeted verification, and record Verlyn work-item/review evidence.

Independent Cross-Agent Review

• Not required for this change - reason: ___
• Completed - reviewer tool: Verlyn workflow gate | findings summary: No blocking findings recorded.
  • Unresolved findings:
    • ___

Rollback Plan

• Revert the bugfix for Fix response finalizer JSON validation failure if the targeted regression returns or new regressions appear.

Handoff Status

• All acceptance criteria satisfied
• Verlyn-managed work items updated
• Outstanding blockers or deferred items noted
• Ready to merge