feat(graphql): Add changeCode field to FlagComparison type

[sentry]

This PR addresses the GraphQLError: Cannot query field 'changeCode' on type 'FlagComparison'. by adding the requested field to the GraphQL schema and implementing its resolver.

Problem:
Clients attempting to query the changeCode field on the FlagComparison type received a GraphQL validation error because the field was not defined in the schema.

Solution:

1. Schema Update: Added changeCode: Float to apps/codecov-api/graphql_api/types/flag_comparison/flag_comparison.graphql.
2. Resolver Implementation: Implemented resolve_change_code in apps/codecov-api/graphql_api/types/flag_comparison/flag_comparison.py.
   • This resolver calculates the difference between head_totals.percentCovered and base_totals.percentCovered for a given FlagComparison.
   • It gracefully handles cases where head_totals, base_totals, or their respective coverage values are None by returning None.

This change allows clients to successfully query for the changeCode on FlagComparison objects.

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Fixes API-ESH

[codecov-notifications]

Codecov Report

❌ Patch coverage is 18.18182% with 9 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...aphql_api/types/flag_comparison/flag_comparison.py	18.18%	9 Missing ⚠️

❌ Your patch check has failed because the patch coverage (18.18%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

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[codecov]

Codecov Report

❌ Patch coverage is 18.18182% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.81%. Comparing base (d8942db) to head (94bc880).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...aphql_api/types/flag_comparison/flag_comparison.py	18.18%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1871      +/-   ##
==========================================
- Coverage   91.83%   91.81%   -0.02%     
==========================================
  Files        1332     1332              
  Lines       51830    51841      +11     
  Branches     1647     1647              
==========================================
+ Hits        47598    47600       +2     
- Misses       3911     3920       +9     
  Partials      321      321              

Flag	Coverage Δ
apiunit	94.71% <18.18%> (-0.05%)	⬇️

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