[router][grpc] Add more mcp test cases to responses api

[CatherineSue]

Motivation

This PR adds test_mixed_mcp_and_function_tools test cases to mcp test.
And it enables TestGrpcBackend in responses api tests.

Modifications

Accuracy Tests

Benchmarking and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.

[gemini-code-assist]

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