4.x: Streamable.timer to dispose the executor/scheduler upon finish

[akarnokd]

Forgot to cleanup the resource (Scheduler.Worker, Future) upon finish executing. Just setting it to DISPOSED can lead to worker leak or task occupying the executor until their timer runs out.

Trivial fix.

Testing it, however, needed a TestScheduler update:

• worker dispose should remove outstanding work in the shared queue for that specific worker
• ability to check how many outstanding tasks are still in the scheduler.

Reported as ??? security vulnerability ??? via AI tool and zero human understanding

@August829

RxJava-11: StreamableTimer.finish() Bypasses DisposableHelper.dispose(), Leaving the Real Scheduled Task/Thread Running

🛑 Do not report memory or resource leaks as security vulnerabilities 🛑

These are ordinary oversights or corner case bugs, in an alpha library, that goes up to maven central so that the brave can look at it, and allows me to use it in other companion libraries already.

I've updated the SECURITY.md to emphasize it again; do not report leaks as security vulnerabilities. Post an issue or PR.

Resolves #8285

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.586 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

SECURITY.md: 4.613
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableTimer.java: 4.533
src/main/java/io/reactivex/rxjava4/schedulers/TestScheduler.java: 4.533
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableTimerTest.java: 4.664

✅ 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 98.61%. Comparing base (61b02bb) to head (9437898).
⚠️ Report is 1 commits behind head on 4.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x    #8286   +/-   ##
=========================================
  Coverage     98.60%   98.61%           
+ Complexity     7148     7147    -1     
=========================================
  Files           824      824           
  Lines         50789    50791    +2     
  Branches       6910     6911    +1     
=========================================
+ Hits          50080    50086    +6     
- Misses          477      481    +4     
+ Partials        232      224    -8     

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