feat(core): add support for MCP 2026 stateless draft and auto-negotiation fallback

[anubhav756]

Description

This PR standardizes the Python SDK's version opt-in strategy for the upcoming stateless draft and introduces protocol auto-negotiation fallback to ensure smooth backward compatibility with older servers.

Note

This PR is stacked on top of the test infrastructure changes in #706.

Changes:

• Protocol Constants (protocol.py):
  • Introduces the Protocol.MCP_DRAFT constant mapped to "DRAFT-2026-v1" to give developers a safe way to opt into testing the draft spec without hardcoding ephemeral version strings.
  • Maintains Protocol.MCP_LATEST mapped to the stable 2025-11-25 release to ensure production workloads remain unaffected.
• Auto-Negotiation Fallback (client.py & mcp.py):
  • Implements fallback protocol negotiation during client initialization.
  • If a client requests the draft protocol but the older server returns a single older version string (instead of throwing a -32004 UnsupportedProtocolVersionError), the transport layer now safely raises a ProtocolNegotiationError.
  • The client catches this error and cleanly re-initializes the session using the older server's preferred protocol.
• E2E Testing (test_e2e_mcp.py):
  • Adds test_protocol_fallback_e2e to verify smooth protocol negotiation downgrades against older servers. Leveraging the dual-server harness from test: upgrade integration harnesses for dual-server execution #706, this test proves that a client requesting MCP_DRAFT successfully falls back when hitting port 5000, but correctly utilizes the draft protocol on port 5001.

[Yuan325]

Please remove this PR, or move some of the part from #704 to here. There seems to be some redundant code (updating MCP_LATEST to the draft specs) that will be reverted in the next PR.

[anubhav756]

Please remove this PR, or move some of the part from #704 to here. There seems to be some redundant code (updating MCP_LATEST to the draft specs) that will be reverted in the next PR.

Merged #704 in this one.

[Yuan325]

Approve with a comment for clarification~

[twishabansal]

nit: Could you link to the MCP draft/source of truth for this PR?

[anubhav756]

nit: Could you link to the MCP draft/source of truth for this PR?

This PR actually isn't directly implementing an MCP SEP, but adapting to recent changes from the server. However, the most relevant SEP is https://modelcontextprotocol.io/seps/2575-stateless-mcp#unsupported-protocol-versions that has the error codes that we are handling here. Hope that helps.