Upgrade hive_ce_generator to analyzer 14

[Rexios80]

Summary

• Fixes Support analyzer >= 13.X #308
• Upgrade hive_ce_generator to analyzer ^14.0.0 and rexios_lints ^18.1.0
• Vendor the small source_helper APIs in use so that dependency no longer blocks the upgrade
• Replace json_serializable codegen for RegistrarIntermediate with hand-written JSON methods and drop json_annotation

Test plan

• dart pub get in hive_generator (and example) resolves on analyzer 14
• dart analyze / tests for hive_generator pass
• Generator still produces expected adapters / registrar output in the example

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.59%. Comparing base (3d5832e) to head (93e4575).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #311   +/-   ##
=======================================
  Coverage   93.59%   93.59%           
=======================================
  Files          64       64           
  Lines        2622     2622           
=======================================
  Hits         2454     2454           
  Misses        168      168           

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