improve error messages for common config mistakes

[AllanChain]

This covers the main confusing cases: missing required key, bad module paths, and malformed CLI overrides like key without =.

Unused config keys now produce direct JaQMC errors instead of a generic stop or mixed raw exceptions.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.41176% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.45%. Comparing base (6dee554) to head (d80e249).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/jaqmc/utils/config.py	77.41%	3 Missing and 4 partials ⚠️
src/jaqmc/utils/module_resolver.py	95.45%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   89.47%   89.45%   -0.02%     
==========================================
  Files         163      163              
  Lines        8775     8833      +58     
  Branches      933      946      +13     
==========================================
+ Hits         7851     7902      +51     
- Misses        602      605       +3     
- Partials      322      326       +4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.