Serialize subscriptions/listen writes so the graceful result is the final message

## Motivation and Context

On graceful teardown (`transport.close`), each open `subscriptions/listen` stream receives
its `SubscriptionsListenResult` response before closing, signaling a clean end the client can distinguish
from an abrupt disconnect. But a delivery that snapshotted the stream from the registry
before teardown cleared it performs its write outside the lock, so a change notification could land after the final result.

Each subscription entry now carries a per-stream write mutex. Teardown marks the entry closed and writes
the result under that mutex; a racing delivery either lands before the result or observes `closed` and skips,
keeping the graceful result the stream's final message. The mutex is per stream and held only around the single write call,
so deliveries to different streams stay as parallel as before, and no path nests it inside the transport mutex
(delivery snapshots release `@mutex` first, and the write-error cleanup takes `@mutex` only after the write mutex is released),
ruling out lock-order inversions. Keepalive pings stay outside the mutex: they are SSE comment frames, which clients ignore by
specification, so one landing after the result is harmless.

## How Has This Been Tested?

With a new regression test pinning the race (an entry captured before teardown and re-presented to delivery afterwards writes
nothing after the result), the full suite, RuboCop, and the conformance suite, all green.

## Breaking Changes

None. The write mutex is internal to the listen registry; the wire only gains the ordering guarantee.

lib/mcp/server/transports/streamable_http_transport.rb

@@ -169,7 +169,8 @@ module MCP
           @allowed_origins = Array(allowed_origins).map(&:downcase).freeze
           @pending_responses = {}
 
-          # Maps a `subscriptions/listen` request id to `{ stream: stream_object, filter: honored_subscription_filter, active: boolean }` (SEP-2575).
+          # Maps a `subscriptions/listen` request id to
+          # `{ stream: stream_object, filter: honored_subscription_filter, active: boolean, write_mutex: Mutex }` (SEP-2575).
           # In-process only; a multi-worker deployment needs an external event bus to fan notifications out across processes,
           # which is a follow-up.
           @listen_subscriptions = {}
@@ -936,7 +937,7 @@ module MCP
                   (@max_listen_subscriptions && @listen_subscriptions.size >= @max_listen_subscriptions)
                 rejected = true
               else
-                @listen_subscriptions[request_id] = { stream: stream, filter: honored, active: false }
+                @listen_subscriptions[request_id] = { stream: stream, filter: honored, active: false, write_mutex: Mutex.new }
               end
             end
 
@@ -1066,24 +1067,32 @@ module MCP
                 uris.is_a?(Array) && uris.include?(uri)
               end
 
-              [request_id, subscription[:stream]] if hit
+              [request_id, subscription] if hit
             end
           end
 
-          matched.each do |request_id, stream|
+          matched.each do |request_id, subscription|
             meta = { RequestEnvelope::SUBSCRIPTION_ID_META_KEY.to_sym => request_id }
             notification_params = (params || {}).merge(_meta: meta)
             notification = { jsonrpc: "2.0", method: method, params: notification_params }
 
             begin
-              send_to_stream(stream, notification)
+              # The per-stream write mutex orders this write against a concurrent graceful teardown:
+              # once teardown has marked the entry closed and written its `SubscriptionsListenResult`,
+              # a delivery that snapshotted the entry before the registry was cleared skips it instead
+              # of writing after the final message.
+              subscription[:write_mutex].synchronize do
+                next if subscription[:closed]
+
+                send_to_stream(subscription[:stream], notification)
+              end
             rescue *STREAM_WRITE_ERRORS => e
               MCP.configuration.exception_reporter.call(
                 e,
                 { subscription_id: request_id, error: "Failed to send notification" },
               )
               remove_listen_subscription(request_id)
-              close_stream_safely(stream)
+              close_stream_safely(subscription[:stream])
             end
           end
         end
@@ -1102,20 +1111,27 @@ module MCP
           end
 
           removed.each do |request_id, subscription|
-            begin
-              send_to_stream(subscription[:stream], {
-                jsonrpc: "2.0",
-                id: request_id,
-                result: {
-                  # `SubscriptionsListenResult` is served at the transport layer and never
-                  # passes through the dispatch path, so the REQUIRED 2026-07-28 `resultType` is
-                  # stamped at its construction site.
-                  resultType: ResultType::COMPLETE,
-                  _meta: { RequestEnvelope::SUBSCRIPTION_ID_META_KEY.to_sym => request_id },
-                },
-              })
-            rescue *STREAM_WRITE_ERRORS
-              nil
+            # Marking the entry closed and writing the result under the stream's write mutex orders
+            # this against in-flight deliveries: each one either lands before the result or observes
+            # `closed` and skips, keeping the graceful result the stream's final message.
+            subscription[:write_mutex].synchronize do
+              subscription[:closed] = true
+
+              begin
+                send_to_stream(subscription[:stream], {
+                  jsonrpc: "2.0",
+                  id: request_id,
+                  result: {
+                    # `SubscriptionsListenResult` is served at the transport layer and never
+                    # passes through the dispatch path, so the REQUIRED 2026-07-28 `resultType` is
+                    # stamped at its construction site.
+                    resultType: ResultType::COMPLETE,
+                    _meta: { RequestEnvelope::SUBSCRIPTION_ID_META_KEY.to_sym => request_id },
+                  },
+                })
+              rescue *STREAM_WRITE_ERRORS
+                nil
+              end
             end
             close_stream_safely(subscription[:stream])
           end