[ISSUE #10810] Redact authentication signatures in audit logs

[Aias00]

What

• Mask authentication request signatures before writing audit logs.
• Keep only a short prefix/suffix for long signatures and fully redact short or blank signatures.
• Add tests for short and long signature masking.

Why

Authentication signatures are security-sensitive request material. Audit logs should preserve useful diagnostics without emitting the raw signature value.

Validation

• JAVA_HOME=$(/usr/libexec/java_home -v 1.8) mvn -pl auth -am -DskipITs -DskipCheckStyle -Dtest=DefaultAuthenticationProviderTest -DfailIfNoTests=false test
• git diff --check

Closes #10810

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.22%. Comparing base (eddb235) to head (b93cf20).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop   #10811      +/-   ##
=============================================
- Coverage      48.33%   48.22%   -0.11%     
+ Complexity     13524    13491      -33     
=============================================
  Files           1380     1380              
  Lines         101104   101111       +7     
  Branches       13107    13108       +1     
=============================================
- Hits           48864    48763     -101     
- Misses         46281    46359      +78     
- Partials        5959     5989      +30     

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