fix(test): use atomic writes for code get --out to prevent file truncation

[crypticsaiyan]

Summary

Previously, running test code get --out <path> would open and immediately truncate the destination file before issuing the network request. If the request subsequently failed (e.g., network error, 4xx, NOT_FOUND) or hit the "no code generated yet" branch (which writes nothing and exits 0), the user's pre-existing file was left completely empty, with no way to recover the original contents.

Note: The bundle.ts command already correctly mitigates this using an atomic write pattern (writing to a .tmp file and renaming it only upon success); code get --out had skipped this safety mechanism.

The Fix

Implemented the atomic write pattern to ensure safe file operations. openOutputFile now writes to a sibling temporary file (.{name}.tmp-{uuid}) instead of directly targeting the user's requested path.

This ensures closeOutputFile(sink, commit) renames the temporary file onto the real --out path only when content is successfully downloaded and written. On any failure, or if the test has no code yet, the temporary file is safely discarded, and the user's original file is left completely untouched.

Testing & Validation

Regression Test: Added new test cases in src/commands/test.test.ts to simulate failure and empty states. Now, file content is preserved in the following cases (previously emptied):

• Failing fetch: Simulated a network error with a pre-existing --out file.
• Empty content: Simulated a "No code generated yet" response with a pre-existing --out file.

CI Gates: All local checks pass.

• npm run lint & npm run format:check
• npm run typecheck
• npm test (1449 tests passing)
• npm run test:coverage (≥80% threshold met)

Files Changed:

• src/commands/test.ts
• src/commands/test.test.ts