Server-initiated notifications from tool handlers are not delivered to the client

[devcrocod]

• Target: server
• Component: sdk-server
• Scenarios: tools-call-with-logging, tools-call-with-progress, tools-call-sampling, tools-call-elicitation,
  elicitation-sep1034-defaults, elicitation-sep1330-enums

Root cause

Notifications and requests sent from a tool handler (notification(), sendLoggingMessage(), createMessage()) have no relatedRequestId. The Streamable HTTP transport therefore routes them to the standalone SSE stream instead of the SSE stream associated with the originating tools/call POST request. The conformance runner only reads from the request-specific stream, so it never receives these messages (notifications are lost, requests time out)

[kpavlov]

See #599