Fix prewarm memory leak and race condition

[romtsn]

📜 Description

Refactored the prewarm() method in SentryExecutorService to address critical bugs. The method now safely schedules dummy tasks, explicitly cancels them, and clears the queue within a single, atomic operation.

💡 Motivation and Context

This change fixes multiple issues in the prewarm() method:

• Memory Leak & Integer Overflow: The previous use of Long.MAX_VALUE for delay caused an integer overflow, and the scheduled dummy tasks were not properly cleared by executorService.getQueue().clear() as they reside in the DelayQueue, leading to a memory leak of these tasks.
• Race Condition & Task Loss: Submitting the scheduling and clearing operations as two separate tasks created a race condition, potentially leading to the incorrect clearing and loss of legitimate tasks submitted by other threads.

The fix ensures dummy tasks are scheduled with a safe delay, explicitly cancelled using their ScheduledFuture references, and all operations are combined into a single, atomic task to prevent race conditions and task loss.

💚 How did you test it?

• Performed syntax validation of the modified code via direct Java compilation.
• Manually reviewed the code changes for correctness and adherence to existing patterns.
• Verified that the changes align with the intent of the existing SentryExecutorServiceTest.kt test for prewarm().

📝 Checklist

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

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## Unreleased

- Fix prewarm memory leak and race condition ([#4611](https://github.com/getsentry/sentry-java/pull/4611))

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Generated by 🚫 dangerJS against 1d9fb9a

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Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	470.00 ms	501.00 ms	31.00 ms
Size	1.58 MiB	2.09 MiB	522.23 KiB

Baseline results on branch: rz/perf/enhanced-executor-service

Startup times

Revision	Plain	With Sentry	Diff
018e963	419.07 ms	463.71 ms	44.64 ms

App size

Revision	Plain	With Sentry	Diff
018e963	1.58 MiB	2.09 MiB	521.49 KiB