ref(grouping): Small enhancements refactors

[lobsterkatie]

This is a series of small refactors to enhancements code, extracted from upcoming PRs in order to make them easier to review. No behavior changes.

• Break the base64 helper functions up into more steps.
• Pull the rust exception data into its own variable before passing it to the rust enhancer.
• Remove Enhancements.as_dict and the EnhancementsDict type. These used to be used in the /grouping-enhancements endpoint, but it was removed 4 years ago in ref(grouping): No longer expose enhancement bases for upgrading #24854.
• Clean up enhancement tests (mostly adding/organizing helper functions and fixtures).

[codecov]

Codecov Report

Attention: Patch coverage is 99.14530% with 1 line in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
tests/sentry/grouping/test_enhancer.py	99.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #90101      +/-   ##
==========================================
+ Coverage   85.44%   87.72%   +2.28%     
==========================================
  Files       10247    10248       +1     
  Lines      578490   578483       -7     
  Branches    22696    22696              
==========================================
+ Hits       494269   507486   +13217     
+ Misses      83778    70554   -13224     
  Partials      443      443