fix: resolve Gemini $ref schemas, fix XML/JSON extraction mismatch, handle null content

- Resolve $ref/$defs in JSON Schema before passing to Gemini (root cause of 0 issues)
- Change extraction prompt from XML to JSON format to match extract_json()
- Handle null content/parts in Gemini responses gracefully
- Add response text and extraction details to Axiom telemetry events

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>