4.x: Single API reduction: merge

[akarnokd]

Simplify API by changing the number of overloads of various operators.

Single

• merge and mergeDelayError -> merge with SingleMergeConfig configuration record.
• mergeArray and mergeArrayDelayError -> mergeArray with SingleMergeConfig configuration record.
• Removed the 2, 3 and 4 argument merge|mergeDelayError variants, use mergeArray

Related: #8173

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.825 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Single.java: 4.845
src/main/java/io/reactivex/rxjava4/core/config/SingleConcatConfig.java: 4.762
src/main/java/io/reactivex/rxjava4/core/config/SingleConcatEagerConfig.java: 4.774
src/test/java/io/reactivex/rxjava4/core/config/SingleConcatConfigEagerTest.java: 4.950
src/test/java/io/reactivex/rxjava4/core/config/SingleConcatConfigTest.java: 4.971
src/test/java/io/reactivex/rxjava4/internal/operators/single/SingleConcatArrayDelayErrorTest.java: 4.906
src/test/java/io/reactivex/rxjava4/internal/operators/single/SingleConcatArrayEagerDelayErrorTest.java: 4.907
src/test/java/io/reactivex/rxjava4/internal/operators/single/SingleConcatDelayErrorTest.java: 4.796
src/test/java/io/reactivex/rxjava4/internal/operators/single/SingleConcatEagerTest.java: 4.510
src/test/java/io/reactivex/rxjava4/validators/ParamValidationCheckerTest.java: 4.834

✅ 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 98.33%. Comparing base (76b01f9) to head (27daaf2).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8168      +/-   ##
============================================
- Coverage     98.34%   98.33%   -0.02%     
  Complexity     6945     6945              
============================================
  Files           777      778       +1     
  Lines         49056    49047       -9     
  Branches       6595     6595              
============================================
- Hits          48244    48228      -16     
- Misses          622      627       +5     
- Partials        190      192       +2     

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