Describe the bug
The current spec indicates that the version should be negotiated in the initial initialize request: https://modelcontextprotocol.io/specification/2025-03-26/basic/lifecycle#version-negotiation

However, Session Management specifies that a Session-Id header MAY be returned: https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#session-management

The Tool protocol doesn't convey any information about which version was previously negotiated. The server doesn't have a way to know which version was previously negotiated.

Separately, the auth part of the spec mentions a "MCP-Protocol-Version" that SHOULD be included: https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization#2-3-1-server-metadata-discovery-headers

To Reproduce
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Expected behavior
The negotiated version in the "MCP-Protocol-Version" should be changed to MUST, and should be mentioned in the "version negotiation" part of the spec.

Logs
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Additional context
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