fix: harden flaky tests and cover native google thought signatures (#23457) (thanks @echoVic)

src/agents/transcript-policy.test.ts

@@ -19,6 +19,10 @@ describe("resolveTranscriptPolicy", () => {
       modelApi: "google-generative-ai",
     });
     expect(policy.sanitizeToolCallIds).toBe(true);
+    expect(policy.sanitizeThoughtSignatures).toEqual({
+      allowBase64Only: true,
+      includeCamelCase: true,
+    });
   });
 
   it("enables sanitizeToolCallIds for Mistral provider", () => {