fix(slack): guard against undefined text in includes calls during mention handling

src/auto-reply/reply/mentions.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+import { stripStructuralPrefixes } from "./mentions.js";
+
+describe("stripStructuralPrefixes", () => {
+  it("returns empty string for undefined input at runtime", () => {
+    expect(stripStructuralPrefixes(undefined as unknown as string)).toBe("");
+  });
+
+  it("returns empty string for empty input", () => {
+    expect(stripStructuralPrefixes("")).toBe("");
+  });
+
+  it("strips sender prefix labels", () => {
+    expect(stripStructuralPrefixes("John: hello")).toBe("hello");
+  });
+
+  it("passes through plain text", () => {
+    expect(stripStructuralPrefixes("just a message")).toBe("just a message");
+  });
+});

src/auto-reply/reply/mentions.ts

@@ -111,6 +111,9 @@ export function matchesMentionWithExplicit(params: {
 }
 
 export function stripStructuralPrefixes(text: string): string {
+  if (!text) {
+    return "";
+  }
   // Ignore wrapper labels, timestamps, and sender prefixes so directive-only
   // detection still works in group batches that include history/context.
   const afterMarker = text.includes(CURRENT_MESSAGE_MARKER)