fix(google): preserve request timeout with http options

[he-yufeng]

Summary

• preserve conn_options.timeout when callers pass custom Google http_options
• copy request-local HTTP options before adding LiveKit headers, avoiding mutation of the caller's object
• add a Google LLM regression test for custom headers plus fallback timeout propagation

Fixes #5972

To verify

• PYTHONPATH=livekit-agents;livekit-plugins\livekit-plugins-google python -m pytest tests\test_plugin_google_llm.py::TestCachedContentRequestSuppression::test_request_merges_timeout_into_caller_http_options tests\test_plugin_google_llm.py::TestCachedContentRequestSuppression::test_request_includes_system_instruction_and_tools_when_no_cache -q
• python -m py_compile livekit-plugins\livekit-plugins-google\livekit\plugins\google\llm.py tests\test_plugin_google_llm.py
• python -m ruff check livekit-plugins\livekit-plugins-google\livekit\plugins\google\llm.py tests\test_plugin_google_llm.py
• python -m ruff format --check livekit-plugins\livekit-plugins-google\livekit\plugins\google\llm.py tests\test_plugin_google_llm.py
• git diff --check

Local test environment note: upgraded livekit-protocol to the project-declared >=1.1.13 and installed the Google plugin's declared speech/texttospeech dependencies so the existing Google plugin tests could import cleanly.

[devin-ai-integration]

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