Agents: scope sanitizeUserFacingText rewrites to errorContext

Squash-merge #12988.

Refs: #12889 #12309 #3594 #7483 #10094 #10368 #11317 #11359 #11649 #12022 #12432 #12676 #12711

src/agents/tools/sessions-helpers.test.ts

@@ -30,4 +30,14 @@ describe("extractAssistantText", () => {
     };
     expect(extractAssistantText(message)).toBe("Hi there");
   });
+
+  it("rewrites error-ish assistant text only when the transcript marks it as an error", () => {
+    const message = {
+      role: "assistant",
+      stopReason: "error",
+      errorMessage: "500 Internal Server Error",
+      content: [{ type: "text", text: "500 Internal Server Error" }],
+    };
+    expect(extractAssistantText(message)).toBe("HTTP 500: Internal Server Error");
+  });
 });