Support multiple MCP protocol versions

[maxisbey]

Summary

Handle different MCP protocol versions gracefully, particularly for proxy/gateway use cases where servers may be on older spec versions.

Problem

• Proxies connecting to third-party MCP servers may encounter older protocol versions
• Per spec, if mcp-protocol-version header is not set in HTTP transport, SDK should assume 2025-03-26
• Currently unclear how version mismatches are handled

Goal

• Graceful handling of protocol version negotiation
• Clear behavior when connecting to servers on different versions
• Support for proxies that need to bridge different versions

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_{AI Disclaimer}

[maxisbey]

This landed as part of v2, which is now released — closing. Feel free to reopen if there's something still outstanding.