Rename authenticated user transport variables

[cwperks]

Follow-up to #6419. Removes legacy Subject-oriented local variable names left behind after authenticated users began being stored directly as User instances.

This renames transport-local variables and updates comments in SecurityInterceptor and SecurityRequestHandler. Existing ThreadContext keys and transport header constants remain unchanged for compatibility.

Validation

• ./gradlew spotlessApply
• ./gradlew test --tests org.opensearch.security.transport.SecurityInterceptorTests --tests org.opensearch.security.transport.RestoringTransportResponseHandlerTest
• git diff --check

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🧪 No relevant tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.41%. Comparing base (d1b600b) to head (a141a2f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...search/security/transport/SecurityInterceptor.java	87.50%	0 Missing and 1 partial ⚠️
...rch/security/transport/SecurityRequestHandler.java	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6432      +/-   ##
==========================================
- Coverage   75.51%   75.41%   -0.10%     
==========================================
  Files         456      456              
  Lines       30282    30282              
  Branches     4574     4574              
==========================================
- Hits        22866    22838      -28     
- Misses       5284     5311      +27     
- Partials     2132     2133       +1     

Files with missing lines	Coverage Δ
...search/security/transport/SecurityInterceptor.java	79.34% <87.50%> (ø)
...rch/security/transport/SecurityRequestHandler.java	59.28% <88.88%> (ø)

... and 6 files with indirect coverage changes

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