fix: skip idle terminal.onEvent subscription and release pruned terminal buffers

- connection.ts: Only subscribe to client.terminal.onEvent when
  applyTerminalEvent handler is provided. No production caller passes
  this handler, so the subscription was a no-op wasting WebSocket traffic.

- terminalSessionState.ts: Reset buffer atoms to EMPTY_TERMINAL_BUFFER_STATE
  when metadata snapshot pruning removes terminals no longer reported by
  the server. Releases up to 512KB of retained string data per pruned
  terminal session on long-running clients.

Author: cursoragent

apps/web/src/environments/runtime/connection.ts

@@ -143,11 +143,13 @@ export function createEnvironmentConnection(
     },
   );
 
-  const unsubTerminalEvent = input.client.terminal.onEvent(
-    (event: Parameters<Parameters<WsRpcClient["terminal"]["onEvent"]>[0]>[0]) => {
-      input.applyTerminalEvent?.(event, environmentId);
-    },
-  );
+  const unsubTerminalEvent = input.applyTerminalEvent
+    ? input.client.terminal.onEvent(
+        (event: Parameters<Parameters<WsRpcClient["terminal"]["onEvent"]>[0]>[0]) => {
+          input.applyTerminalEvent!(event, environmentId);
+        },
+      )
+    : () => undefined;
 
   const cleanup = () => {
     disposed = true;

packages/client-runtime/src/terminalSessionState.ts

@@ -402,6 +402,10 @@ export function createTerminalSessionManager(config: TerminalSessionManagerConfi
 
       for (const key of Object.keys(next)) {
         if (!retainedKeys.has(key)) {
+          const removedEntry = next[key];
+          if (removedEntry) {
+            setBuffer(removedEntry.target, EMPTY_TERMINAL_BUFFER_STATE);
+          }
           delete next[key];
         }
       }