test: avoid /dev/null in command tests

[han-dreamer]

Summary

Replace hard-coded /dev/null test fixtures with temporary files created at runtime.

This makes the affected command tests portable across Windows and Unix-like environments.

Changes

• Use a temporary fixture file in file upload dry-run output test.
• Use a temporary lyrics file in music generate validation test.
• Clean up temporary directories after each test.
• Reuse a typed stdout capture helper instead of assigning process.stdout.write with any.

Why

/dev/null is available on Unix-like systems but not on Windows. When running the test suite on Windows, these tests fail before reaching the intended assertions.

Testing

• npx tsc --noEmit
• npx eslint test/commands/file/upload.test.ts test/commands/music/generate.test.ts
• npx bun test
  Result: 354 pass, 0 fail

Notes

I also tried the local smoke compile with npx bun build src/main.ts --compile --outfile dist/mmx-test, but Bun failed on this Windows environment with failed to copy bun executable into temporary file: ENOENT. The full test suite and typecheck pass locally.

[TumCucTom]

good call — /dev/null in test code is the kind of thing that works on the original author's machine and silently breaks Windows contributors. small and targeted, exactly the right scope.

if you want a follow-up: a mkdtemp-style helper exported from test/utils/ would let future test files that need a scratch dir stop reinventing the pattern. the per-file cleanup is fine for now though.

approving.