- Add jsonschema crate for schema validation in extract_json
- On parse failure (invalid JSON): retry once with identical request
- On schema mismatch (valid JSON, wrong schema): fail immediately — no retry
- validate_schema() produces concise error with field path from instance_path()
- Add SequenceMockProvider to testing.rs for first-fail/second-success tests
- Fix env var test flakiness: mark env_model_override as ignored