More fixes for functional group handling

[mrbean-bremen]

• fixed combining of errors in shared and per/frame groups
• improved handling of conditions that refer to tags in other func group sequences
• fixes Functional-group conditions misevaluate when controlling tags exist only per-frame #226

As discussed in the issue, in case some tags that are expected in the shared group now also are looked up in the first frame of the per-frame group. This may still not be the optimal solution, but should handle most cases.

Tasks

• Unit tests added that reproduce the issue or prove feature is working
• Fix or feature added
• Entry to release notes added
• Pre-commit CI shows no errors
• Unit tests passing

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.02326% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.89%. Comparing base (e8644e2) to head (abd5f16).
⚠️ Report is 150 commits behind head on main.

Files with missing lines	Patch %	Lines
dicom_validator/validator/iod_validator.py	93.02%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
- Coverage   89.82%   88.89%   -0.94%     
==========================================
  Files          15       21       +6     
  Lines        1632     2107     +475     
==========================================
+ Hits         1466     1873     +407     
- Misses        166      234      +68     

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