Update MCP protocol validation and defaults

[soodoku]

Summary

• prefer the 2025-11-25 MCP protocol version while retaining compatibility with 2025-06-18 in HTTP transport validation and examples
• align server protocol defaults to the latest MCP version when the SDK is unavailable
• update protocol compliance documentation and expectations to match the new default version

Testing

• python -m compileall rmcp/transport/http.py rmcp/core/server.py tests/integration/protocol/test_mcp_protocol_compliance.py

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Respect negotiated MCP version for 2025-06-18 clients

When the MCP SDK is missing, _PROTOCOL_VERSION now defaults to 2025-11-25, yet the HTTP transport still accepts 2025-06-18 (see _validate_protocol_version). An older client that sends initialize with protocolVersion/header 2025-06-18 will now get an initialize response advertising 2025-11-25 because _handle_initialize always returns _PROTOCOL_VERSION, so strict clients will reject the handshake as a version mismatch despite being explicitly allowed. The server should either echo the client’s requested version when supported or fail negotiation instead of unilaterally upgrading.

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[chatgpt-codex-connector]

Summary

• Added a shared SUPPORTED_PROTOCOL_VERSIONS constant to capture the preferred and compatible MCP protocol versions for negotiation. rmcp/core/server.pyL35-L50
• Introduced protocol negotiation in initialize handling so responses echo a supported client version instead of always upgrading to the latest default. rmcp/core/server.pyL461-L532

Testing

• ✅ python -m compileall rmcp/transport/http.py rmcp/core/server.py tests/integration/protocol/test_mcp_protocol_compliance.py

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