fix: accept grounding chunk metadata

[he-yufeng]

Summary

Fixes #2510.

Some Vertex batch responses with Google Search grounding include groundingMetadata.groundingChunks[].chunkMetadata. The SDK currently rejects that payload because GroundingChunk forbids unknown fields and does not declare chunk_metadata.

This adds the field to both the model and TypedDict shape so the response can validate without dropping the metadata.

Testing

python -m pytest google\genai\tests\types\test_types.py::test_generate_content_response_accepts_grounding_chunk_metadata -q
python -m pytest google\genai\tests\types\test_types.py -q
python -m py_compile google\genai\types.py google\genai\tests\types\test_types.py
git diff --check

I also ran python -m pyink --check google\genai\types.py google\genai\tests\types\test_types.py; it wants to reformat the existing generated/style-preserved files wholesale, so I did not apply it.

[he-yufeng]

Rebased this branch onto current main and pushed the updated head 1bb2fbb.

Validation:

• python -m pytest google\genai\tests\types\test_types.py -q -k "grounding or metadata or chunk" -> passed, 1 passed
• python -m py_compile google\genai\types.py google\genai\tests\types\test_types.py -> passed
• git diff --check upstream/main..HEAD -> passed