Add uvx as a supported package manager so that `uvx` commands are
routed through safe-chain's MITM proxy for malware detection, just
like `uv`. Previously, `uvx` bypassed all safe-chain protections.
The uvx package manager reuses the existing uv command runner since
uvx is functionally equivalent to `uv tool run`.
Fixes#268
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
PDM is a modern Python package manager using pyproject.toml (PEP 621).
Uses the same MITM-only proxy approach as poetry/uv/pipx — all malware
detection and minimum package age enforcement happens at the proxy layer
by intercepting PyPI requests.
Before this change, if a package manager was not installed, safe-chain still
sets the function and when invoked, the wrapper will invoke safe-chain, which
will exit with error code 127 when it fails to invoke the wrapped command. As an
example (with a shell prompt that shows $? when non-zero):
```
$ type -f pip
bash: type: pip: not found
1$ pip
127$
```
With this patch, the wrapper first checks for the existence of the wrapped
command (ignoring functions), and if no such command exists, it instructs the
shell to invoke it anyway. This results in the shell failing to find the
command, and reporting an error as if the wrapper function wasn't there:
```
$ source init-posix.sh
$ type -f pip
bash: type: pip: not found
1$ pip
Command 'pip' not found, but can be installed with:
sudo apt install python3-pip
127$
```
