fix: tighten isSilentReplyText to match whole-text only

The suffix regex matched NO_REPLY at the end of any response,
suppressing substantive replies when models (e.g. Gemini 3 Pro)
appended NO_REPLY to real content.

Replace prefix+suffix regexes with a single whole-string match.
Only responses that are entirely the silent token (with optional
whitespace) are now suppressed.

Add unit tests for the fix.

Fixes #19537

src/auto-reply/tokens.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect } from "vitest";
+import { isSilentReplyText } from "./tokens.js";
+
+describe("isSilentReplyText", () => {
+  it("returns true for exact token", () => {
+    expect(isSilentReplyText("NO_REPLY")).toBe(true);
+  });
+
+  it("returns true for token with surrounding whitespace", () => {
+    expect(isSilentReplyText("  NO_REPLY  ")).toBe(true);
+    expect(isSilentReplyText("\nNO_REPLY\n")).toBe(true);
+  });
+
+  it("returns false for undefined/empty", () => {
+    expect(isSilentReplyText(undefined)).toBe(false);
+    expect(isSilentReplyText("")).toBe(false);
+  });
+
+  it("returns false for substantive text ending with token (#19537)", () => {
+    const text = "Here is a helpful response.\n\nNO_REPLY";
+    expect(isSilentReplyText(text)).toBe(false);
+  });
+
+  it("returns false for substantive text starting with token", () => {
+    const text = "NO_REPLY but here is more content";
+    expect(isSilentReplyText(text)).toBe(false);
+  });
+
+  it("returns false for token embedded in text", () => {
+    expect(isSilentReplyText("Please NO_REPLY to this")).toBe(false);
+  });
+
+  it("works with custom token", () => {
+    expect(isSilentReplyText("HEARTBEAT_OK", "HEARTBEAT_OK")).toBe(true);
+    expect(isSilentReplyText("Checked inbox. HEARTBEAT_OK", "HEARTBEAT_OK")).toBe(false);
+  });
+});