test(chat): add extractAgentText coverage — 6 cases

Covers the 4 unique branches not exercised by extractTextsFromParts:
artifacts[0].parts fallback, status.message.parts fallback, bare-string
passthrough, and the try/catch sentinel on null input.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/tabs/chat/__tests__/message-parser.test.ts

@@ -281,6 +281,41 @@ describe("extractTextsFromParts", () => {
   });
 });
 
+describe("extractAgentText", () => {
+  it("extracts from task.parts via extractTextsFromParts", () => {
+    const task = { parts: [{ kind: "text", text: "Agent reply" }] };
+    expect(extractAgentText(task)).toBe("Agent reply");
+  });
+
+  it("falls back to artifacts[0].parts when task.parts is absent", () => {
+    const task = {
+      artifacts: [{ parts: [{ kind: "text", text: "Artifact text" }] }],
+    };
+    expect(extractAgentText(task)).toBe("Artifact text");
+  });
+
+  it("falls back to status.message.parts when both parts and artifacts are absent", () => {
+    const task = {
+      status: { message: { parts: [{ kind: "text", text: "Status text" }] } },
+    };
+    expect(extractAgentText(task)).toBe("Status text");
+  });
+
+  it("returns the task as-is when it is a bare string", () => {
+    expect(extractAgentText("Bare string response")).toBe("Bare string response");
+  });
+
+  it("returns sentinel when nothing extractable is present", () => {
+    expect(extractAgentText({})).toBe("(Could not extract response text)");
+  });
+
+  it("returns sentinel for null input (caught by try/catch)", () => {
+    expect(extractAgentText(null as unknown as Record<string, unknown>)).toBe(
+      "(Failed to parse response)"
+    );
+  });
+});
+
 describe("extractFilesFromTask", () => {
   it("pulls A2A file parts out of a result", () => {
     const task = {