test: add regression test for error-success mask detection

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/tests/error-success-mask.test.ts

@@ -0,0 +1,37 @@
+/**
+ * error-success-mask.test.ts — #3664
+ *
+ * Verify that the agent-end-recovery error handler detects when errorMessage
+ * is uninformative (e.g. "success", "ok", "unknown") and falls back to
+ * extracting the real error from the assistant message text content.
+ */
+
+import { describe, test } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const sourceFile = join(__dirname, "..", "bootstrap", "agent-end-recovery.ts");
+
+describe("error-success mask detection (#3664)", () => {
+  const source = readFileSync(sourceFile, "utf-8");
+
+  test("detects useless errorMessage values with regex", () => {
+    assert.match(source, /success\|ok\|true\|error\|unknown/i);
+  });
+
+  test("extracts display message from content text block", () => {
+    assert.match(source, /textBlock/);
+    assert.match(source, /\.text\.slice\(0,\s*300\)/);
+  });
+
+  test("classifies using rawErrorMsg, not displayMsg", () => {
+    assert.match(source, /classifyError\(rawErrorMsg/);
+  });
+
+  test("references issue #3588 in comments", () => {
+    assert.match(source, /#3588/);
+  });
+});