feat(sdk): add termination deferral to fix parallel minSuccessful race condition

[ParidelPooya]

Implement three-layer defense mechanism to prevent race conditions when parallel execution with minSuccessful completes while branches have pending callbacks.

Layer 1: Active Operations Tracking

• Add ActiveOperationsTracker to track in-flight async operations
• Wrap checkpoint operations with increment/decrement
• Defer termination until active operations complete

Layer 2: Pending Completions Check

• Track operations being checkpointed in pendingCompletions Set
• Add hasPendingAncestorCompletion() to detect in-flight completions
• Check both completed status and pending completions

Layer 3: Delayed Ancestor Check

• Use setImmediate before checking ancestor status
• Gives checkpoint queue time to process
• Ensures ancestor status is current when checked

Changes:

• Add ActiveOperationsTracker class and tests
• Enhance terminate() with three-layer protection
• Add hasPendingAncestorCompletion() to CheckpointHandler
• Update ExecutionContext with activeOperationsTracker
• Add example: parallel with 100 branches, minSuccessful:1
• Add comprehensive documentation

Fixes race where:

• Branch completes, starts checkpointing
• Parallel sees minSuccessful reached, returns
• Other branches try to terminate
• Termination interrupted checkpoint
• Parallel failed instead of succeeded

Now:

• Pending branches detect ancestor finished
• Skip termination, return never-resolving promise
• Parallel succeeds correctly

Tests: All 691 SDK tests pass, 100-branch example passes in 254ms

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[anthonyting]

Can we create an issue to follow-up to investigate the issues with polling and using setImmediate?