Upgrade rexios_lints to 19

[Rexios80]

Summary

• Upgrade rexios_lints to 19 in hive, hive_generator, their examples, and the storage benchmark
• Await futures that trip async_return_with_no_await and fix the one simple_directive_paths hit

Test plan

• dart analyze --fatal-infos in hive and hive_generator on Dart 3.13
• hive tests still pass (CI test-hive still strips rexios_lints and runs on Dart 3.4)

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.48%. Comparing base (e98c319) to head (565aa32).

Files with missing lines	Patch %	Lines
hive/lib/src/box_collection/box_collection.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
+ Coverage   93.40%   93.48%   +0.07%     
==========================================
  Files          64       64              
  Lines        2625     2623       -2     
==========================================
  Hits         2452     2452              
+ Misses        173      171       -2     

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