Bugfix/uv shim fallback

[davebelais]

This pull request improves the fallback logic when the uv executable is present but cannot be run, ensuring more robust handling of package management commands. The main changes include updating both the implementation and documentation to treat OSError (e.g., due to a dangling shebang, corrupted binary, or permission error) as a trigger for falling back to pip, not just CalledProcessError. The test suite is also extended to cover these scenarios.

Fallback Logic Improvements:

• The fallback from uv to pip in _iter_pip_list and _install_requirement_string now triggers on both CalledProcessError and OSError, ensuring that any execution failure of uv (including unrunnable binaries) results in a retry with pip. [1] [2]
• Added the _raise_oserror helper to ensure that if uv was not attempted and pip itself fails with OSError, the error is propagated without modification, preserving prior behavior. [1] [2]

Documentation Updates:

• The fallback design spec is updated to clarify that OSError (not just nonzero exit codes) from uv triggers a fallback to pip, and describes the resulting behavior in detail. [1] [2]

Testing Enhancements:

• Added _write_broken_uv and _write_installable_package test helpers to simulate both nonzero-exit and unrunnable uv executables, and to create real installable test packages.
• New tests verify that fallback to pip works when uv is unrunnable (OSError), and that the error handling and recovery logic behave as intended in all cases. [1] [2] [3]

Miscellaneous:

• Bumped the project version to 1.4.3 in pyproject.toml.
• Minor code comment and formatting improvements for clarity.

These changes make the fallback mechanism more robust and ensure that package operations do not fail unexpectedly when uv is present but unusable.