test(signal): dedupe receive event fixtures and add mention clamp case

src/signal/monitor/event-handler.test-harness.ts

@@ -33,3 +33,17 @@ export function createBaseSignalEventHandlerDeps(
     ...overrides,
   };
 }
+
+export function createSignalReceiveEvent(envelopeOverrides: Record<string, unknown> = {}) {
+  return {
+    event: "receive",
+    data: JSON.stringify({
+      envelope: {
+        sourceNumber: "+15550001111",
+        sourceName: "Alice",
+        timestamp: 1700000000000,
+        ...envelopeOverrides,
+      },
+    }),
+  };
+}