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.