feat: optional session store (resumabillity support)

[glicht]

Motivation and Context

In horizontally-scaled deployments or after restarting a server, MCP clients use Mcp-Session-Id to resume sessions. When routed to a different server instance or after a restart, the session is unknown and clients get a 404, forcing re-initialization.

This PR adds an optional SessionStore trait that persists InitializeRequestParams after a successful handshake. When a request arrives at an instance without a matching in-memory session, the store is consulted and the session is transparently restored by replaying initialize.

The SessionManager trait gains a restore_session method with a default no-op implementation. Custom session manager implementations can override it to integrate with their own logic if needed. The built-in LocalSessionManager has an implementation that re-creates the in-memory session worker.

Additionally to provide indication to ServerHandler implementation that a call to initialize and on_initialized is as a result of a restore, a marker SessionRestoreMarker is added to the context.extensions so implementors can act appropriately when a session is restored.

The feature is opt-in. Configurations without a session_store configured are unaffected.

Note: for full resumability and multi-server (behind a load balancer) support there is need to also implement an Event store so events aren't lost. This is being discussed at: #330

How Has This Been Tested?

Added a new integration test suite at: test_streamable_http_session_store.rs with three test scenarios.

Breaking Changes

None. Existing logic remains if session_store is not configured (defaults to None).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• [ x I have added or updated documentation as needed

Additional context

Related to #330