agent_session: rename CustomEvent -> DebugMessage; drop type field

.changeset/rename-custom-event-to-debug-message.md

@@ -0,0 +1,14 @@
+---
+"@livekit/protocol": patch
+---
+
+agent_session: rename `CustomEvent` → `DebugMessage`; drop `type` field, keep only `payload`
+
+Renames the agent-session event added in #1588 before any consumer ships it. The
+message is repositioned as an internal debug/trace channel surfaced only to the
+debugger/recorder, not to user code, so the `type` discriminator was unnecessary
+— callers just emit a JSON `payload`.
+
+Wire-level: `AgentSessionEvent.custom_event` (field 21) → `AgentSessionEvent.debug_message`
+(same field number 21, same type slot, no schema-compat concerns since nothing
+has been built against it yet downstream).

protobufs/agent/livekit_agent_session.proto

@@ -183,10 +183,10 @@ message AgentSessionUsage {
   repeated ModelUsage model_usage = 1;
 }
 
-// Application-defined event carried over the remote session wire.
-message CustomEvent {
-  string type = 1;
-  google.protobuf.Struct payload = 2;
+// Debug/trace payload carried over the remote session wire.
+// Internal: surfaced only to the debugger/recorder, not to user code.
+message DebugMessage {
+  google.protobuf.Struct payload = 1;
 }
 
 message AgentSessionEvent {
@@ -262,7 +262,7 @@ message AgentSessionEvent {
     AmdPrediction amd_prediction = 18;
     EotPrediction eot_prediction = 19;
     FunctionToolsStarted function_tools_started = 20;
-    CustomEvent custom_event = 21;
+    DebugMessage debug_message = 21;
   }
 }