Add support for model aifs-subs and aifs-subs-crps

[dsarmany]

Description

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.

[Copilot]

Pull request overview

Adds support for additional AIFS model identifiers in the metkit MARS→GRIB pipeline by extending the background-process table mapping and exposing the new model names in the metkit language schema.

Changes:

• Extends BackgroundProcess enum and string↔enum mappings to include aifs-subs and aifs-subs-crps.
• Updates the set of allowed model values in share/metkit/language.yaml.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/metkit/mars2grib/backend/tables/backgroundProcess.h	Adds new background-process enum values and updates the mapping functions/documentation.
share/metkit/language.yaml	Allows aifs-subs and aifs-subs-crps as valid model values.

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[Copilot]

@dsarmany I've opened a new pull request, #180, to work on those changes. Once the pull request is ready, I'll request review from you.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 20.00000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.61%. Comparing base (e0c9b28) to head (838d1bc).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...etkit/mars2grib/backend/tables/backgroundProcess.h	20.00%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #179      +/-   ##
===========================================
+ Coverage    62.54%   62.61%   +0.06%     
===========================================
  Files          303      303              
  Lines        11548    11554       +6     
  Branches      1029     1029              
===========================================
+ Hits          7223     7234      +11     
+ Misses        4325     4320       -5     

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[tweska]

Looks good to me, just need to run clang-format to make the CI happy. :^)