fix: avoid mutating raw schemas for responses tools

[pragnyanramtha]

Summary

Copy raw function schemas before formatting them for OpenAI Responses tools. The previous formatter added type: function directly to RawFunctionTool.info.raw_schema, which leaked provider-specific fields back into the stored tool schema and allowed callers to mutate nested schema state through the returned provider payload.

Validation

• .venv/bin/python -m pytest tests/test_tools.py::TestProviderSchemaFormatting -q -> 2 passed
• .venv/bin/python -m pytest tests/test_tools.py::TestNoParametersSchema tests/test_tools.py::TestProviderSchemaFormatting -q -> 4 passed
• .venv/bin/ruff format --check livekit-agents/livekit/agents/llm/_provider_format/openai.py tests/test_tools.py -> passed
• .venv/bin/ruff check livekit-agents/livekit/agents/llm/_provider_format/openai.py tests/test_tools.py -> passed
• git diff --check -> passed

Note: I used a narrow local venv with livekit-agents, livekit-plugins-openai, pytest, pytest-asyncio, and ruff installed, rather than syncing the full workspace.

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[devin-ai-integration]

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