fix cmin for low voltage 10% tolerance

[nis-nurr]

This pull request updates the handling of c factor in low voltage grids and synchronizes the expected results in several short-circuit calculation tests to match the new logic. The main change is the use of correct minimum c factor (c_min) for lv_tol_percentage 10 instead of a fixed value.

Voltage tolerance calculation improvements:

• Updated _add_c_to_ppc in pandapower/build_bus.py to use c_min variable for LV bus voltage coefficient, replacing the previous fixed value and ensuring consistency with IEC 60909.

Test coverage enhancements:

• Added a new minimal test test_iec60909_on_single_branch in pandapower/test/shortcircuit/test_sc_single_bus.py to verify short-circuit calculations on a single branch for both 6% and 10% voltage tolerances, which we also verified manually.

See also the discussion in #2846

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.86%. Comparing base (2efb4f9) to head (c410057).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2913   +/-   ##
========================================
  Coverage    71.86%   71.86%           
========================================
  Files          351      351           
  Lines        37856    37858    +2     
========================================
+ Hits         27206    27208    +2     
  Misses       10650    10650           

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