* google gen ai format error fix
* fix(core): add $ref safety net, warning log, and unit tests for Google schema resolution
- Add `$ref` to unsupported_keys in `_clean_google_ai_schema_properties` so unresolvable refs (e.g. `#/properties/...` style) are stripped as a safety net instead of crashing the Google SDK
- Add warning log when `_resolve_json_schema_refs` encounters a ref it cannot resolve
- Deduplicate the `#/$defs/` and `#/definitions/` resolution branches
- Add 11 unit tests covering: single/multiple $defs, nested refs, refs in anyOf/allOf, array items, definitions key, unresolvable refs, and the full resolve+clean pipeline
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Co-Authored-By: Letta <noreply@letta.com>
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Co-authored-by: Letta <noreply@letta.com>
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