refactor(map): Fix null results, abstract completion logic

[ayushiahjolia]

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Issue Link, if available

#39

Description

• Fix NPE in ChildContextOperation.checkpointSuccess when map iterations return null by adding a null guard on the serialized payload before calling getBytes().
• Make canComplete() and validateItemCount() abstract in ConcurrencyOperation so Map and Parallel each own their completion semantics. Move Parallel-specific logic (fail-fast on any failure when minSuccessful == -1) into ParallelOperation. - Remove unused minSuccessful/toleratedFailureCount/failureRateThreshold fields from the base class.
• Remove dead runUntilCompleteOrSuspend stub from BaseDurableOperationTest.
• Add testMapWithNullResults integration test.
• Update design.md with ConcurrencyOperation, MapOperation, ParallelOperation, and ChildContextOperation in the architecture diagram and package structure.

Demo/Screenshots

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Checklist

• I have filled out every section of the PR template
• I have thoroughly tested this change

Testing

Unit Tests

Have unit tests been written for these changes? Updated required tests.

Integration Tests

Have integration tests been written for these changes? Added missing tests.

Examples

Has a new example been added for the change? (if applicable) N/A