Chardata covcheck

[pp-mo]

Closes #6921
Now targets FEATURE_chardata

Addressing coverage holes noted there

The actual coverage achieved is now pretty much 100%. See this check on this draft PR

Change exists on parent branch, will no longer appear here when that is merged

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.37%. Comparing base (eaf35e9) to head (5ca639f).

Files with missing lines	Patch %	Lines
...ib/iris/fileformats/netcdf/_bytecoding_datasets.py	78.57%	3 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##           FEATURE_chardata    #7137      +/-   ##
====================================================
+ Coverage             90.24%   90.37%   +0.13%     
====================================================
  Files                    92       92              
  Lines                 25151    25775     +624     
  Branches               4698     4901     +203     
====================================================
+ Hits                  22697    23294     +597     
- Misses                 1682     1691       +9     
- Partials                772      790      +18     

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[trexfeathers]

Thanks @pp-mo!