CodesHandle::getBytes: use length instead of size to determine array size

[pgeier]

Description

This PR fixes getting CodesHandle::getBytes in metkit/codes/api/CodesAPI.cc.
The way how the number of bytes is determined has been unclear and differs from the way arrays are retrieved.
Although bytes are returned as an array, they are handled similar to string and hence have there size described through the codes_get_length call instead of codes_get_size.

The highlevel API takes care of these subtle differences. One usecase of the problem has been in the GRIBDecoder where uuidOfHGrid was accessed.

See related issue: https://jira.ecmwf.int/browse/ECC-2166

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.57576% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.73%. Comparing base (33f1681) to head (df5ee2a).

Files with missing lines	Patch %	Lines
src/metkit/codes/BUFRDecoder.cc	0.00%	11 Missing ⚠️
src/metkit/codes/GRIBDecoder.cc	81.25%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #129      +/-   ##
===========================================
- Coverage    70.81%   70.73%   -0.09%     
===========================================
  Files          132      132              
  Lines         8182     8207      +25     
  Branches       784      785       +1     
===========================================
+ Hits          5794     5805      +11     
- Misses        2388     2402      +14     

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