#9730 fix(search): preserve query attribution on fused results

Summary

• Preserve original native-query indexes on fused Search results so callers can attribute a document to the searches or folders that returned it.
• Keep the optional metadata in the Search MCP response and generated Assistant contract. Ranking, permissions, pagination and provider requests are unchanged.

Type of Change

• Bug fix

Testing

• Regression assertions failed before each production fix and pass afterward.
• 38 real MCP/DB integration checks and 100 focused application/MCP tests pass.
• Root lint, type-check, all 63 audits, test-type ratchet and full test suite pass.
• Model evaluation and harness corrections are tracked in the companion worker PR. The final evaluation retains one model input/retry failure; no zero-error model claim.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

Companion: simstudioai/mothership#930