pip: uv doesn't support --format=columns/freeze/--not-required

[PLNech]

Context

Found by agentic fuzzer (FUZZ-007 residual). RTK auto-detects uv and uses it instead of pip, but uv pip list doesn't support:

• --format=columns (uv only supports --format=json and --format=freeze as of recent versions)
• --format=freeze (may work in newer uv versions)
• --not-required flag

When these flags are passed, uv silently produces empty output or errors.

Options

1. Detect unsupported flags and fall back to system pip instead of uv
2. Passthrough to system pip when format flags other than --format=json are present
3. Document as known limitation (uv compat)

Severity

Low - affects edge cases. pip list and pip list --outdated (the common paths) work fine with uv.