fix: Remove hdate from variable keys

[gmertes]

Description

The checkpoint variables metadata might contain an hdate key in the data request, if the model was trained on hindcast data.

This key needs to be skipped when constructing the initial condition request and writing to grib.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.07%. Comparing base (90728d5) to head (3b6c477).
Report is 76 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
- Coverage   98.03%   96.07%   -1.97%     
==========================================
  Files           3        3              
  Lines          51       51              
==========================================
- Hits           50       49       -1     
- Misses          1        2       +1     

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