everything: remove disconnected sessions from resource subscriptions

[vishalhabib99]

subscriptions (Map<uri, Set<sessionId>>) only ever had entries removed via an explicit resources/unsubscribe call. A session that disconnects without unsubscribing stayed in every Set it had joined for the life of the process, since cleanup(sessionId) stopped logging/update intervals and task-store timers but never touched subscriptions.

Adds removeSubscriber(sessionId), which drops the session from every URI's subscriber set and removes any URI entry left with no subscribers, and calls it from cleanup().

Fixes #4710

(Re-opened as a fresh PR — the original #4711 was closed automatically when my fork was deleted; same fix, no changes.)

[vishalhabib99]

Pulled 8e2f0fe, 109 passing vs 107 on base (d73f99e), build and tsc --noEmit on src/everything both clean.

Added two regression tests for removeSubscriber: one confirming a removed session stops receiving simulated resource-update notifications, one confirming removing that session doesn't affect other sessions still subscribed to the same URI. Verified both actually catch the regression — stubbing removeSubscriber to a no-op locally fails the first test, restoring the fix passes it again.

[vishalhabib99]

Rebased onto latest main and CI is green (all Python/TypeScript build+test jobs passing). Added two regression tests for the removeSubscriber fix (a removed session stops receiving resource-update notifications; removing one session doesn't affect others still subscribed to the same URI) — verified both actually catch the regression by stubbing the fix back out locally.

Ready for review whenever you have a chance.