Stateless HTTP transport and routing

[g2vinay]

Make HTTP mode compliant with the stateless protocol and explicit routing contract.

Work items:

1. Verify that the HTTP transport emits and accepts MCP-Protocol-Version, Mcp-Method, and Mcp-Name correctly.
2. Add negative tests for header/body disagreement.
3. Review the Azure HTTP startup path in core/Microsoft.Mcp.Core/src/Areas/Server/Commands/ServiceStartCommand.cs and servers/Azure.Mcp.Server/src/Program.cs for any transport-specific assumptions that only existed because of session affinity.
4. Decide whether core/Microsoft.ModelContextProtocol.HttpServer.Distributed/src/SessionAffinityEndpointFilter.cs should be:
   • retired from the MCP path,
   • kept only for legacy compatibility experiments, or
   • refactored into a generic state-routing package with no MCP-specific contract names.
5. Remove any deployment guidance that tells operators to treat session affinity as a protocol requirement.

Acceptance criteria:

• HTTP requests are routable without any session store or sticky routing requirement.
• Gateway/operator docs describe header-based routing instead of session-based routing.
• Tests cover both success and rejection of malformed headers.