fix(ai): classify Google thoughtSignature as thinking

Google streaming may emit thoughtSignature without thought=true (including empty-text signature-only parts). Treat non-empty thoughtSignature as thinking to avoid leaking reasoning into normal text and retain signature across streaming deltas. Add unit test coverage.

packages/ai/test/google-thinking-signature.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it } from "vitest";
+import { isThinkingPart, retainThoughtSignature } from "../src/providers/google-shared.js";
+
+describe("Google thinking detection (thoughtSignature)", () => {
+	it("treats part.thought === true as thinking", () => {
+		expect(isThinkingPart({ thought: true, thoughtSignature: undefined })).toBe(true);
+	});
+
+	it("treats a non-empty thoughtSignature as thinking even if thought is missing", () => {
+		// This is the bug: some backends omit `thought: true` but still include `thoughtSignature`
+		expect(isThinkingPart({ thought: undefined, thoughtSignature: "opaque-signature" })).toBe(true);
+		expect(isThinkingPart({ thought: false, thoughtSignature: "opaque-signature" })).toBe(true);
+	});
+
+	it("does not treat empty/missing signatures as thinking if thought is not set", () => {
+		expect(isThinkingPart({ thought: undefined, thoughtSignature: undefined })).toBe(false);
+		expect(isThinkingPart({ thought: false, thoughtSignature: "" })).toBe(false);
+	});
+
+	it("preserves the existing signature when subsequent deltas omit thoughtSignature", () => {
+		const first = retainThoughtSignature(undefined, "sig-1");
+		expect(first).toBe("sig-1");
+
+		const second = retainThoughtSignature(first, undefined);
+		expect(second).toBe("sig-1");
+
+		const third = retainThoughtSignature(second, "");
+		expect(third).toBe("sig-1");
+	});
+
+	it("updates the signature when a new non-empty signature arrives", () => {
+		const updated = retainThoughtSignature("sig-1", "sig-2");
+		expect(updated).toBe("sig-2");
+	});
+
+	// Note: signature-only parts (empty text + thoughtSignature) are handled by isThinkingPart via thoughtSignature presence.
+});