fix(everything): drop disconnected session from subscriptions map

[AbhiPra24]

Description

Fixes #4710

When a client session closes or disconnects, its session ID remained in the subscriptions map indefinitely. This patch introduces cleanupSubscriptions(sessionId) to remove the disconnected session from tracked URI subscriptions (and prune empty URIs from the map) and invokes it during server cleanup (cleanup(sessionId)). Also cleans up empty subscriber sets on explicit unsubscribe requests.

Changes

• In src/everything/resources/subscriptions.ts: Added cleanupSubscriptions to remove all subscriptions for a session ID and delete empty URI entries; deleted empty URI sets in UnsubscribeRequestSchema handler; exported getSubscriptions for inspection/testing.
• In src/everything/server/index.ts: Added cleanupSubscriptions(sessionId) to cleanup callback returned by createServer.
• Added unit tests in src/everything/__tests__/resources.test.ts and src/everything/__tests__/server.test.ts to verify session subscription cleanup on disconnect.

Verification

• npm --workspace=@modelcontextprotocol/server-everything test passed (108 tests passing).
• npm run build passed.