[svdconv] Correct enum combo width warning

[thorstendb-ARM]

Fixes

Report the actual field width and the six-bit combo limit instead of the incorrect 256-element maximum. Add integration coverage for the six- and seven-bit boundary.
See #2569

Changes

• Message M227

Checklist

• 🤖 This change is covered by unit tests (if applicable).
• 🤹 Manual testing has been performed (if necessary).
• 🛡️ Security impacts have been considered (if relevant).
• 📖 Documentation updates are complete (if required).
• 🧠 Third-party dependencies and TPIP updated (if required).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.31%. Comparing base (fdcbbe9) to head (feedf62).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2584      +/-   ##
==========================================
+ Coverage   72.16%   72.31%   +0.15%     
==========================================
  Files         185      185              
  Lines       30253    30257       +4     
  Branches    17845    17849       +4     
==========================================
+ Hits        21831    21880      +49     
+ Misses       6110     6072      -38     
+ Partials     2312     2305       -7     

Flag	Coverage Δ
svdconv-cov	47.26% <100.00%> (+0.44%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...s/svdconv/SVDGenerator/src/SfdData_SingleItems.cpp	51.36% <100.00%> (+3.69%)	⬆️

... and 6 files with indirect coverage changes

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[github-actions]

Test Results

 4 files  10 suites   0s ⏱️
31 tests 31 ✅ 0 💤 0 ❌
62 runs  62 ✅ 0 💤 0 ❌

Results for commit feedf62.

[jkrech]

LGTM