feat: support external subcommands via PATH (#1343)

[OsamaMIT]

Summary

This PR adds support for external subcommands in the hugr-clibinary, as per the feature request from issue #1343.
Now when you run:

hugr <subcommand> [args…]

if an executable named hugr-<subcommand> is on your PATH, it will be called with the same [args…]. If the binary is missing or fails, you get a clear error and non-zero exit code.

---

Details

Functionality:

• src/main.rs

  1. Checks for no subcommand and errors out.

  2. Builds hugr-<subcommand> from the first arg.

  3. Runs it with std::process::Command, passing the remaining args.

  4. On error:

     • If not found →

       error: no such subcommand: '<subcommand>'.  
       Could not find 'hugr-<subcommand>' in PATH.
       

     • Other I/O errors →

       error: failed to invoke '<executable>': <error>
       

Testing:

• hugr-cli/tests/external.rs

  • test_missing_external_command
    Runs hugr idontexist and checks for a “no such subcommand” error.

  • test_external_command_invocation

    1. Creates a temp dir with a dummy hugr-dummy script that echoes its args and exits 42.
    2. Prepends that dir to PATH.
    3. Runs hugr dummy foo bar and checks it prints dummy called: foo bar and exits 42.

• Adds tempfile to dev-dependencies in Cargo.toml for these tests.

---

Closes #1343

[codecov]

Codecov Report

Attention: Patch coverage is 73.91304% with 6 lines in your changes missing coverage. Please review.

Project coverage is 82.15%. Comparing base (ede8e7b) to head (6671809).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-cli/src/main.rs	73.91%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2278      +/-   ##
==========================================
- Coverage   82.15%   82.15%   -0.01%     
==========================================
  Files         239      239              
  Lines       42989    43010      +21     
  Branches    38901    38922      +21     
==========================================
+ Hits        35319    35334      +15     
- Misses       5679     5685       +6     
  Partials     1991     1991              

Flag	Coverage Δ
python	85.34% <ø> (ø)
rust	81.81% <73.91%> (-0.01%)	⬇️

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[aborgna-q]

Awesome, thanks!

I just have a couple comments, to make it pass the CI checks

[aborgna-q]

Awesome, thanks for contributing!