test: simplify SingleRetryTest callables

[puneetdixit200]

Part of #8080.

This updates SingleRetryTest as one small cleanup chunk by replacing old anonymous Callable and Predicate implementations with lambdas. The test behavior is unchanged, and the now-unused imports are removed.

Tests run:

• JAVA_HOME=/tmp/rxjava-jdk26/Contents/Home ./gradlew --no-daemon test --tests io.reactivex.rxjava4.single.SingleRetryTest
• JAVA_HOME=/tmp/rxjava-jdk26/Contents/Home ./gradlew --no-daemon checkstyleTest
• git diff --check

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 5.065 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

.github/workflows/gradle_branch.yml: 5.078
.github/workflows/gradle_pr.yml: 4.928
.github/workflows/gradle_release.yml: 5.203
.github/workflows/gradle_snapshot.yml: 5.051

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.77%. Comparing base (3bda03c) to head (e6cef13).

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x    #8120   +/-   ##
=========================================
  Coverage     98.76%   98.77%           
- Complexity     6871     6874    +3     
=========================================
  Files           769      769           
  Lines         48659    48659           
  Branches       6516     6516           
=========================================
+ Hits          48059    48061    +2     
+ Misses          459      457    -2     
  Partials        141      141           

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