Update notifications design doc with implementation plan

[david-martin]

Summary

Update the notifications design doc to include a detailed implementation plan that enables the various notification-related features to be implemented in a coordinated way.

Background

The current design doc provides a good overview of the notification architecture and distinguishes between:

• State Change Events (broadcast) - already implemented
• Client-Specific Events (per-client) - partially implemented (progress updates work, elicitations in progress)

However, there are several notification features still to be implemented (see sub-issues of parent #600)

Scope

The design doc update should include:

1. Implementation phases/ordering - Which features should be built first to enable others
2. Architectural decisions for per-client notifications:
   • Connection management strategy
   • Authentication handling for per-client backend connections
   • Resource overhead mitigation
3. Request ID mapping strategy - Detailed approach for elicitations and other client-specific requests
4. Dependency graph between notification features
5. Open questions resolution - Address the open questions section in the current doc
6. Updated sequence diagrams as needed

Deliverables

• Updated docs/design/notifications.md with implementation plan section
• Clear dependencies between notification-related issues

Related Issues

• Support for MCP logging #407 - Support for MCP logging
• Support for cancellations via notifications #422 - Support for cancellations via notifications
• Elicitation Support #349 - Elicitation Support
• Test for multiple long-running tools at the same time (e.g. server1 slow) to make sure notifications arrive at the client without being buffered #371 - Test for multiple long-running tools notifications
• Per-client notifications infrastructure #599 - Per-client notifications infrastructure
• Support for resource subscriptions #597 - Support for resource subscriptions

Tracking

• MCP Notifications Feature Support #600 - MCP Notifications Feature Support (parent tracking issue)