test(tts): add test for missing channel fail-fast

Covers the new no_channel early return when TTS tool is called 
without a bound delivery channel.

Part of #43483

src/agents/tools/tts-tool.test.ts

@@ -9,8 +9,14 @@ const { createTtsTool } = await import("./tts-tool.js");
 describe("createTtsTool", () => {
   it("uses SILENT_REPLY_TOKEN in guidance text", () => {
     const tool = createTtsTool();
-
     expect(tool.description).toContain("QUIET_TOKEN");
     expect(tool.description).not.toContain("NO_REPLY");
   });
+
+  it("returns error when no delivery channel is available", async () => {
+    const tool = createTtsTool();
+    const result = await tool.execute("test-call", { text: "hello world" });
+    expect(result.content[0].text).toContain("requires a bound channel");
+    expect(result.details.error).toBe("no_channel");
+  });
 });