Fix Pinot client scheme property merge

[wolfkill]

Summary

• Preserve a previously configured JsonAsyncHttpPinotClientTransportFactory scheme when connection properties omit scheme
• Keep the default http fallback if the configured scheme is null
• Add a regression test for the setScheme + withConnectionProperties path

Fixes #14500.

Testing

• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home ./mvnw -pl pinot-clients/pinot-java-client -am -Dtest=JsonAsyncHttpPinotClientTransportTest -Dsurefire.failIfNoSpecifiedTests=false test

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.38%. Comparing base (9f8eb1a) to head (6883c30).

Files with missing lines	Patch %	Lines
...ient/JsonAsyncHttpPinotClientTransportFactory.java	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18605      +/-   ##
============================================
+ Coverage     64.37%   64.38%   +0.01%     
  Complexity     1137     1137              
============================================
  Files          3335     3335              
  Lines        206065   206065              
  Branches      32136    32136              
============================================
+ Hits         132649   132672      +23     
+ Misses        62760    62746      -14     
+ Partials      10656    10647       -9     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-21	64.38% <33.33%> (+0.01%)	⬆️
temurin	64.38% <33.33%> (+0.01%)	⬆️
unittests	64.38% <33.33%> (+0.01%)	⬆️
unittests1	56.77% <ø> (+<0.01%)	⬆️
unittests2	36.95% <33.33%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[wolfkill]

Superseded by #18607.