fix(google): preserve FunctionTool schemas for text API

[jstar0]

Summary

Fixes #7170.

Google text requests now emit a FunctionTool's complete JSON Schema through parameters_json_schema, preserving additionalProperties for free-form object parameters. Google Live requests continue to use the existing simplified parameters representation because that API does not support parameters_json_schema.

Verification

• uv run --frozen pytest tests/test_tools.py tests/test_schema_gemini.py -q — 119 passed
• uv run --frozen ruff check livekit-agents/livekit/agents/llm/_provider_format/google.py tests/test_tools.py
• uv run --frozen ruff format --check livekit-agents/livekit/agents/llm/_provider_format/google.py tests/test_tools.py
• git diff --check
• create_tools_config validation confirms the generated FunctionDeclaration retains the nested additionalProperties schema

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.