fix: don't crash on --config-root without --resolve-all-configs

[Gooh456]

yeah this one's just been broken since forever. --config-root on its own throws a raw UnsupportedSettings traceback instead of doing... anything.

Root cause is in main.py: config_root only gets popped out of config_dict inside the if resolve_all_configs: branch. So if you pass --config-root without also passing --resolve-all-configs, it's still sitting in the dict when it gets splatted into Config(**config_dict), and Config has no idea what config_root is, so it errors out.

The help text for the flag literally says it's "used with the --resolve-all-configs flag" - so without that flag it should just be a no-op, not a crash. Fix is to pop it unconditionally, same as resolve_all_configs itself already is a few lines up.

Verified against main: reverted my change locally and reproduced the exact traceback from #1990 (isort.exceptions.UnsupportedSettings: ... - config_root = ..). With the fix it runs fine.

Added a regression test in tests/unit/test_main.py that calls --config-root without --resolve-all-configs and just checks it doesn't blow up.

Fixes #1990

[DanielNoord]

We should give a warning (by raising an error) that this config isn't useful without resolve_all_configs. See other examples in the codebase.

[Gooh456]

good call. added that, exits with a clear message now instead of the confusing UnsupportedSettings crash. same sys.exit("Error: ...") pattern used a few lines up for the no-paths case. updated the old test to expect the error and added one for config-root + resolve-all-configs together still working.

[DanielNoord]

This can remain on line 1051 now

[Gooh456]

yeah good catch, moved it back inline. no reason to hold it in a separate var anymore.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.33%. Comparing base (3d72652) to head (ee9ba80).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2597      +/-   ##
==========================================
+ Coverage   99.30%   99.33%   +0.03%     
==========================================
  Files          41       41              
  Lines        3150     3155       +5     
  Branches      680      682       +2     
==========================================
+ Hits         3128     3134       +6     
  Misses         14       14              
+ Partials        8        7       -1     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.