feat: add MCP proxy pattern convenience function (#12)

[dgenio]

Description

Implements the mcp_proxy() convenience function that enables bidirectional message forwarding between two MCP transports, porting the TypeScript proxy pattern to the Python SDK.

Changes

• Added mcp_proxy() context manager in src/mcp/shared/proxy.py
• Implements bidirectional message forwarding using anyio task groups
• Supports optional error handling with sync/async callbacks
• Automatically closes both transports when one closes
• Comprehensive test coverage in tests/shared/test_proxy.py

Features

• Bidirectional forwarding: Messages flow in both directions simultaneously
• Error handling: Optional onerror callback (supports both sync and async)
• Automatic cleanup: When one transport closes, the other is also closed
• Exception handling: Properly handles SessionMessage and Exception objects from read streams
• Resource management: Uses context manager pattern for proper cleanup

Usage Example

from mcp.shared.proxy import mcp_proxy

async with mcp_proxy( transport_to_client=(client_read, client_write), transport_to_server=(server_read, server_write), onerror=lambda e: logger.error(f"Proxy error: {e}"), ): # Proxy is active, forwarding messages bidirectionally await some_operation()

Both transports are closed when exiting the context## Testing

• Unit tests cover bidirectional forwarding, error handling, stream cleanup, and edge cases
• All tests pass with proper async/await patterns
• Follows existing test patterns in the repository

Related

Closes #12

Type of Change

• [x] New feature (non-breaking change which adds functionality)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

Checklist

• [x] Code follows the repository's style guidelines
• [x] Tests added/updated for new functionality
• [x] All tests pass locally
• [x] Linting and type checking pass
• [x] Documentation updated (if applicable)