4.x: Add SchedulerSupport annotation validator

[akarnokd]

Add a validator test that checks if methods with TimeUnit parameter have the expected SchedulerSupport annotation.

Some methods had NONE because it uses the underlying scheduler for timestamps only. Now with this check, we made explicit what exact scheduler is involved regardless if the method runs code on it or not.

Rules:

• If the TimeUnit is declared but no annotation -> report
• If annotation is there but says NONE -> report
• If the Scheduler is there but no annotation -> report
• If annotation is there but is not CUSTOM -> report

Inspired by #8289

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AI disclosure: the firstLine method was suggested by Grok 4.6 Super for the prompt how to get the method's line number out reflectively -> have to parse the class file and lookup in tables.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.59 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Flowable.java: 4.787
src/main/java/io/reactivex/rxjava4/core/Observable.java: 4.764
src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.651
src/test/java/io/reactivex/rxjava4/validators/CheckSchedulerAnnotationsTest.java: 4.161

✅ 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 98.58%. Comparing base (856bb48) to head (9ddbc20).
⚠️ Report is 1 commits behind head on 4.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8292      +/-   ##
============================================
- Coverage     98.61%   98.58%   -0.03%     
+ Complexity     7150     7149       -1     
============================================
  Files           824      824              
  Lines         50791    50791              
  Branches       6911     6911              
============================================
- Hits          50087    50074      -13     
- Misses          482      486       +4     
- Partials        222      231       +9     

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