Support resource subscriptions (#839)

Clients can now subscribe to specific resources and receive targeted
notifications when those resources change. Previously, calling
`notifyResourceUpdated` on the server would broadcast the notification
to every connected client regardless of interest — now only sessions
that have explicitly subscribed to a given resource URI receive the
update, making resource change propagation both correct and efficient.
The subscription lifecycle is fully handled: clients can subscribe and
unsubscribe at any time, and the server cleans up subscription state
when a session closes.

Supersedes #838.
Resolves #837, #776.

---------

Signed-off-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

conformance-tests/conformance-baseline.yml

@@ -2,11 +2,6 @@
 # This file lists known failing scenarios that are expected to fail until fixed.
 # See: https://github.com/modelcontextprotocol/conformance/blob/main/SDK_INTEGRATION.md
 
-server:
-  # Resource subscription not implemented in SDK
-  - resources-subscribe
-  - resources-unsubscribe
-
 client:
   # SSE retry field handling not implemented
   # - Client does not parse or respect retry: field timing