fix(test): prevent flaky tarball test race condition (#1982)

## Summary

The `createTarball` success test writes `output.tar.gz` into the same
temp directory being archived. `tar` sees the file appear while reading
the directory and exits with code 1 ("file changed as we read it"),
causing intermittent CI failures on any PR that rebases onto main.

Fix: write output to a separate temp directory.

## Test plan

- [x] 10/10 debug tests pass
- [x] Race condition eliminated — output dir is separate from source dir

Signed-off-by: Prekshi Vyas <prekshiv@nvidia.com>

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Improved test isolation by writing generated test artifacts to a
separate temporary output directory instead of the archived source
directory.
* Enhanced cleanup to remove the separate output directory (recursive)
after each test, ensuring no leftover artifacts remain.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Prekshi Vyas <prekshiv@nvidia.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: prekshivyas

src/lib/debug.test.ts

@@ -56,13 +56,15 @@ describe("redact", () => {
 
 describe("createTarball", () => {
   let tempDir: string;
+  let outputDir: string;
 
   beforeEach(() => {
     process.exitCode = undefined;
   });
 
   afterEach(() => {
     if (tempDir) rmSync(tempDir, { recursive: true, force: true });
+    if (outputDir) rmSync(outputDir, { recursive: true, force: true });
     process.exitCode = undefined;
   });
 
@@ -77,7 +79,10 @@ describe("createTarball", () => {
   it("creates tarball successfully and returns true for valid output path", () => {
     tempDir = mkdtempSync(join(tmpdir(), "debug-test-"));
     writeFileSync(join(tempDir, "dummy.txt"), "test data");
-    const output = join(tempDir, "output.tar.gz");
+    // Write output to a SEPARATE directory — writing into the source dir
+    // causes tar to see the file changing as it reads, returning exit 1.
+    outputDir = mkdtempSync(join(tmpdir(), "debug-test-out-"));
+    const output = join(outputDir, "output.tar.gz");
     const ok = createTarball(tempDir, output);
     expect(ok).toBe(true);
     expect(process.exitCode).toBeUndefined();