Use NumberFormatException in baggage decoder

[ADITYA-CODE-SOURCE]

Summary

• replace the invalid hex-digit exception in BaggageCodec.digit16(byte) with NumberFormatException
• include byte and character details in the error message
• add a focused regression test for invalid hex input

Fixes #8556

Testing

• ./gradlew :api:all:test --tests "*BaggageCodecTest" --tests "*W3CBaggagePropagatorTest"
• ./gradlew :api:all:spotlessCheck

[jack-berg]

Seems reasonable enough. Rebase on main to get CI to pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.63%. Comparing base (ee9b063) to head (e95d4e7).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8593   +/-   ##
=========================================
  Coverage     91.63%   91.63%           
  Complexity    10326    10326           
=========================================
  Files          1013     1013           
  Lines         27295    27295           
  Branches       3205     3205           
=========================================
+ Hits          25011    25012    +1     
  Misses         1558     1558           
+ Partials        726      725    -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.