Group executable install errors during `uv python install` (#19691)
Report pre-existing unmanaged executables in a single error instead of
one message per executable.
## Summary
When `uv python install` links the default executables (`python`,
`python3`, `python3.x`) and more than one of them already exists as a
file uv doesn't manage, the output currently repeats almost the same
error once per executable.
The change is in `create_bin_links`
(`crates/uv/src/commands/python/install.rs`), which links each
executable into the bin directory by looping over a list of targets.
Instead of pushing an error for every conflicting executable inside the
loop, it now collects them and reports once after the loop:
- 0 conflicts: nothing is reported
- 1 conflict: the original single-line message (unchanged)
- 2+ conflicts: one grouped error with the paths as a bulleted list
Other, unrelated failures (e.g. failing to remove or re-link a file) are
still reported individually.
Resolves #9707
## Test Plan
Added `python_install_multiple_unmanaged_executables`: install the
default executables, replace all three (`python`, `python3`,
`python3.x`) with unmanaged files, reinstall without `--force`, and
assert that a single grouped error is shown and the existing files are
left untouched. The existing single-conflict tests
(`python_install_force`, `python_install_preview`) still pass, so the
one-executable output is unchanged.
```bash
cargo nextest run -p uv --test it python_install
cargo clippy -p uv --all-targets
```
