Add RPC tester interface to example app (#1085)

Adds a new button on the bottom control bar for the "rpc tester":

<img width="100" height="103" alt="Screenshot 2026-05-15 at 11 09 19 AM"
src="https://github.com/user-attachments/assets/5d7db331-f817-4321-9a22-4834d0966612"
/>

When clicked, it opens up a new panel which has two sections:

<img width="508" height="855" alt="Screenshot 2026-05-15 at 11 09 04 AM"
src="https://github.com/user-attachments/assets/0ea49592-18d3-49b9-a603-959a8d8de175"
/>

The top section allows sending RPC messages - select a destination
participant, topic, and enter a payload, and click "Send". Note that
there are two payload presets - `"hello world"` and `"X" * 20000` to
provide some easy to generate data to test some edge cases.

The bottom section lets you configure rpc handlers. Enter a rpc handler
topic and a static response, and submit. Then when this participant
receives a RPC request, the request will be responded to with the static
response. This static response also has the same payload presets as the
request.

> [!WARNING]
> This pull request was LLM generated and has only been lightly reviewed
by a human. The author has tested this and confirms it works in the
happy path, but no other validation has been done.
> 
> A more thorough review of this needs to occur before it could be
merged.

---------

Co-authored-by: Hiroshi Horie <548776+hiroshihorie@users.noreply.github.com>

Author: 1egoman

.changes/harden-rpc-tester

@@ -0,0 +1 @@
+patch type="fixed" "Harden the example RPC tester"

example/lib/widgets/rpc_test_sheet.dart

@@ -28,8 +28,13 @@ class RpcHandlerEntry {
 }
 
 class RpcTestController extends ChangeNotifier {
+  // Oldest invocations are dropped past this point so a chatty peer cannot
+  // grow the log without bound.
+  static const _maxInvocationsPerHandler = 200;
+
   final List<RpcHandlerEntry> _handlers = [];
   Room? _room;
+  bool _disposed = false;
 
   List<RpcHandlerEntry> get handlers => List.unmodifiable(_handlers);
 
@@ -60,21 +65,33 @@ class RpcTestController extends ChangeNotifier {
     }
 
     final entry = RpcHandlerEntry(topic: topic, staticResponse: staticResponse);
+    try {
+      room.registerRpcMethod(topic, (data) async {
+        final bytes = utf8.encode(data.payload).length;
+        entry.invocations.insert(
+          0,
+          RpcInvocationRecord(
+            timestamp: DateTime.now(),
+            byteLength: bytes,
+            payload: data.payload,
+            callerIdentity: data.callerIdentity,
+          ),
+        );
+        if (entry.invocations.length > _maxInvocationsPerHandler) {
+          entry.invocations.removeLast();
+        }
+        if (!_disposed) {
+          notifyListeners();
+        }
+        return entry.staticResponse;
+      });
+    } on Exception {
+      // The room throws when another component already registered this
+      // method. Adding a card for it would let the tester unregister a
+      // handler it does not own.
+      return false;
+    }
     _handlers.add(entry);
-    room.registerRpcMethod(topic, (data) async {
-      final bytes = utf8.encode(data.payload).length;
-      entry.invocations.insert(
-        0,
-        RpcInvocationRecord(
-          timestamp: DateTime.now(),
-          byteLength: bytes,
-          payload: data.payload,
-          callerIdentity: data.callerIdentity,
-        ),
-      );
-      notifyListeners();
-      return entry.staticResponse;
-    });
     notifyListeners();
     return true;
   }
@@ -97,15 +114,14 @@ class RpcTestController extends ChangeNotifier {
   // is needed. Skip notifyListeners to avoid rebuild loops with the
   // editing TextField that drives this method.
   void updateStaticResponse(String topic, String response) {
-    final entry = _handlers.firstWhere(
-      (h) => h.topic == topic,
-      orElse: () => throw StateError('topic $topic not registered'),
-    );
+    final entry = _handlers.firstWhere((h) => h.topic == topic);
     entry.staticResponse = response;
   }
 
   @override
   void dispose() {
+    // The flag keeps in flight RPC handlers from notifying after disposal.
+    _disposed = true;
     _unregisterAll();
     _room = null;
     super.dispose();
@@ -193,8 +209,13 @@ class _RpcTestSheetState extends State<RpcTestSheet> {
     super.dispose();
   }
 
+  // The selected identity is only usable while that participant is still in
+  // the room.
+  String? get _validSelectedIdentity =>
+      widget.room.remoteParticipants.values.any((p) => p.identity == _selectedIdentity) ? _selectedIdentity : null;
+
   Future<void> _send() async {
-    final identity = _selectedIdentity;
+    final identity = _validSelectedIdentity;
     final method = _methodCtl.text.trim();
     final local = widget.room.localParticipant;
     if (identity == null || method.isEmpty || local == null) {
@@ -321,7 +342,12 @@ class _RpcTestSheetState extends State<RpcTestSheet> {
               final remotes = widget.room.remoteParticipants.values.toList();
               final identities = remotes.map((p) => p.identity).toList();
               final currentValue = identities.contains(_selectedIdentity) ? _selectedIdentity : null;
+              // The form field only reads initialValue when its state is
+              // created, so key it by the participant list. Otherwise a
+              // selection can outlive its participant and trip the framework
+              // assert that the value must be among the items.
               return DropdownButtonFormField<String>(
+                key: ValueKey(identities.join(',')),
                 initialValue: currentValue,
                 decoration: const InputDecoration(
                   labelText: 'Destination',
@@ -401,7 +427,7 @@ class _RpcTestSheetState extends State<RpcTestSheet> {
 
   bool _canSend() =>
       !_isSending &&
-      _selectedIdentity != null &&
+      _validSelectedIdentity != null &&
       _methodCtl.text.trim().isNotEmpty &&
       widget.room.localParticipant != null;