Fix race conditions in TaskContext and CancellationContext cancellation

[diachenko-mischa]

• Enable pre-execution cancellation in TaskContext by registering a cancellation token in constructor that invokes PreExecuteCancel()
• Clear cancellation token in ~TaskContextImpl to prevent dangling refs
• Fix CancellationContext::CancelOperation() to swap out the token before calling Cancel(), avoiding re-entrancy issues
• Update BlockingCancel test to cover pre-execution cancellation

Relates-To: HERESDK-12253

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.46%. Comparing base (b400ee0) to head (4edc623).
⚠️ Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
...cpp-sdk-core/include/olp/core/client/TaskContext.h	84.00%	1 Missing and 3 partials ⚠️
...pp-sdk-core/src/thread/ThreadPoolTaskScheduler.cpp	94.59%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #1689 +/- ##
==========================================
+ Coverage 80.42% 80.46% +0.04% 
==========================================
Files 351 351 Lines 14221 14280 +59 Branches 1544 1554 +10 ==========================================
+ Hits 11437 11490 +53 
Misses 2156 2156 - Partials 628 634 +6 

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