serve: pin that serve hands begin its own max frame (serve-frame-wiring-unpinned)

A record completed under a wide frame and replayed by a serve at a narrower one must
come back as the typed stand-in; passing begin any other frame (usize::MAX) delivers the
wide record the narrow write cannot carry.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/model-connector/src/serve.rs

@@ -558,6 +558,45 @@ mod tests {
         assert_eq!(backend.executions.load(Ordering::SeqCst), 1);
     }
 
+    #[test]
+    fn serve_measures_a_replayed_record_against_its_own_max_frame() {
+        // A record completed under a wide frame, replayed by a serve at a
+        // narrower one, must come back as the typed stand-in that fits the
+        // narrower frame; handing `begin` any other frame delivers the wide
+        // record, which the narrow write then cannot carry.
+        let store = TestStore::new();
+        let invocation = fresh("request-narrowed");
+        let wide = SizedBackend::new(80_000);
+        let (client, _) = start_with(wide.clone(), &store, vec![caller(CALLER, KEY, 4)]);
+        let first = client.execute(&invocation).unwrap();
+        assert!(!failed_result_size(&first));
+
+        let narrow = 100_000;
+        let narrowed = ServeLimits {
+            max_frame_bytes: narrow,
+            ..limits()
+        };
+        let listener = TcpListener::bind("127.0.0.1:0").unwrap();
+        let endpoint = listener.local_addr().unwrap();
+        let path = store.path.clone();
+        let backend = SizedBackend::new(80_000);
+        let served = backend.clone();
+        thread::spawn(move || {
+            serve(
+                listener,
+                narrowed,
+                vec![caller(CALLER, KEY, 4)],
+                &path,
+                served,
+            )
+        });
+        let client =
+            ConnectorClient::new(endpoint, KEY, narrow, Some(Duration::from_secs(5))).unwrap();
+        let replayed = client.execute(&invocation).unwrap();
+        assert!(failed_result_size(&replayed));
+        assert_eq!(backend.executions.load(Ordering::SeqCst), 0);
+    }
+
     #[test]
     fn serve_refuses_to_start_on_an_unready_backend() {
         let store = TestStore::new();