fix(anthropic): preserve sibling keys beside function_response result

[FOWEPJF255]

Summary

• When Anthropic/Claude serializes a function_response dict that has a non-null result and sibling keys (files, stdout, ...), serialize the full dict instead of only result.
• Keeps single-key {result: x} unwrapping for backward compatibility.
• Adds regression test for Anthropic/Claude: tool result dict with a result key drops every sibling key (only json.dumps(result) reaches the model) #7073.

Test plan

• PYTHONPATH=src pytest tests/unittests/models/test_anthropic_llm.py -k "sibling_keys or traditional_result or dict_result or with_content"

Fixes #7073

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[FOWEPJF255]

Rebased onto latest main and resolved the conflict in anthropic_llm.py:

• Keep exact {"result": value} unwrap behavior from main
• When result has sibling keys, serialize the full dict (issue Anthropic/Claude: tool result dict with a result key drops every sibling key (only json.dumps(result) reaches the model) #7073 / Gemini parity)

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[FOWEPJF255]

Follow-up: the code conflict on main is resolved on this branch. cla/google is still required before review can proceed 鈥?I am completing the Google CLA signing for this account and will ping once the CLA check turns green.