Fix ssc bus not updated by create_continuous_bus_index #3078

[quinnbmccarthy]

Fixes #3078

ssc was missing from element_bus_tuples() in pandapower/toolbox/element_selection.py,
which is the central registry that create_continuous_bus_index (and other toolbox
functions) use to know which element tables have a bus column that needs updating on
reindex. As a result, an ssc's bus value went stale after reindexing while every other
bus-connected element updated correctly.

• Added ("ssc", "bus") to element_bus_tuples()
• Extended test_continuos_bus_numbering to include an ssc element, so the existing
  generic "all element bus references stay valid after reindex" assertion covers it
  (confirmed this test fails on develop without the fix, passes with it)
• Added a changelog entry

[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 72.10%. Comparing base (7b972fc) to head (66ebdc4).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3109   +/-   ##
========================================
  Coverage    72.10%   72.10%           
========================================
  Files          355      355           
  Lines        39398    39398           
========================================
  Hits         28408    28408           
  Misses       10990    10990           

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