Assert single-arg Logger.isEnabled overload in shared default logger test

[thswlsqls]

Summary

• Logger has two stable overloads (since 1.61.0): isEnabled(Severity, Context) and isEnabled(Severity).
• The shared base AbstractDefaultLoggerTest#defaultIsEnabled asserted only the 2-arg overload, so the api/all DefaultLogger 1-arg path was never exercised by any concrete test.
• Add a 1-arg assertion to the base, matching the sibling api/incubator ExtendedDefaultLoggerTest which already asserts both overloads.
• 1-arg overload introduced in Stabilize enabled api #8200 (Stabilize enabled api).

Testing done

• ./gradlew :api:testing-internal:check passed (compile/spotless/errorprone; no concrete tests run there).
• ./gradlew :api:all:test --tests "...DefaultLoggerTest" passed.
• ./gradlew :api:incubator:test --tests "...ExtendedDefaultLoggerTest" passed.
• Green before and after (coverage fill, not a behavior change). No public API change (no apidiff), no user-facing change (no CHANGELOG).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: thswlsqls / name: Eunbin Son (27a5b6d)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (27a5b6d).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8483   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
  Complexity     8579     8579           
=========================================
  Files          1009     1009           
  Lines         28993    28993           
  Branches       3599     3599           
=========================================
  Hits          22839    22839           
  Misses         5311     5311           
  Partials        843      843           

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[thswlsqls]

/easycla

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