Add mcp_proxy for bidirectional message forwarding

[maxisbey]

Adds a convenience function for proxying messages between two MCP transports, enabling bidirectional message forwarding with proper error handling.

Motivation and Context

This implements issue #12 - porting the TypeScript proxy pattern to Python. It's a simpler reimplementation of #1711/#1763 that addresses all review feedback from those PRs.

Key differences from #1763:

• 72 lines vs 144 - simpler design with inline helper
• No pragma: no cover - all code paths are tested
• No timing sleeps in tests - uses anyio.Event for synchronization
• No logging - errors propagate to caller
• Correct stream ownership - proxy doesn't close streams it doesn't own

How Has This Been Tested?

13 unit tests covering:

• Bidirectional forwarding
• Error callback (sync and async)
• Stream closure handling (ClosedResourceError, BrokenResourceError)
• Error callback exceptions (ignored, proxy continues)
• Context exit cleanup

All tests pass with pytest-xdist parallel execution.

Breaking Changes

None - this is a new feature.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Closes #12

Related PRs: #1711, #1763

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