Add hint when Python downloads are disabled (#14522)
Follow-up to https://github.com/astral-sh/uv/pull/14509 to provide the
_reason_ downloads are disabled and surface it as a hint rather than a
debug log.
e.g.,
```
❯ cargo run -q -- run --no-managed-python -p 3.13.4 python
error: No interpreter found for Python 3.13.4 in virtual environments or search path
hint: A managed Python download is available for Python 3.13.4, but the Python preference is set to 'only system'
```
