Add RunInChildContextAsync

[GarrettBeatty]

Summary

• Adds RunInChildContextAsync to IDurableContext with two overloads: returning T and void.
• Adds ChildContextConfig (SubType for observability, ErrorMapping for exception remapping) and ChildContextException for failure surfacing.
• Implements Internal/ChildContextOperation<T> mirroring the Step/Wait pattern: sync-flush CONTEXT START -> run user func -> emit SUCCEED with serialized result, or FAIL with error and throw ChildContextException. Replay: SUCCEEDED returns cached value, FAILED throws (after ErrorMapping), STARTED/PENDING re-runs the func and lets the child's own operations replay from their own checkpoints.
• Result serialization uses the ILambdaSerializer registered on ILambdaContext.Serializer (consistent with StepOperation); no separate AOT overload is needed since the AOT story is owned by the registered serializer (e.g. SourceGeneratorLambdaJsonSerializer<TContext>).
• Extends LambdaDurableServiceClient.MapFromSdkOperation to copy ContextDetails (Result + Error).

This is a building block for upcoming WaitForCallbackAsync, which (per the Java/JS reference SDKs) wraps CreateCallbackAsync + a submitter step inside a child context for observability and clean error mapping.

#2216

Test plan

• 14 new tests in ChildContextOperationTests.cs covering: fresh execution + checkpoint emission, deterministic child operation IDs, replay SUCCEEDED, replay FAILED (with and without ErrorMapping), suspended child (Wait inside child propagates termination), replay STARTED with completed inner step, void overload, type-mismatch non-determinism detection, and SubType propagation.
• 161/161 tests pass on net8.0 and net10.0 (existing Step/Wait tests untouched).
• Production build clean: 0 warnings, 0 errors.

[Copilot]

Pull request overview

Adds a RunInChildContextAsync API to IDurableContext enabling user functions to be executed inside a logical sub-workflow whose result is checkpointed as a CONTEXT operation. The child context shares the parent's state, termination manager, batcher, and Lambda context, but uses a child OperationIdGenerator so its operation IDs are deterministically namespaced. Failures are surfaced via a new ChildContextException, optionally remapped via ChildContextConfig.ErrorMapping. This is positioned as a building block for a future WaitForCallbackAsync.

Changes:

• New ChildContextOperation<T> mirroring the Step/Wait pattern with replay branches for SUCCEEDED, FAILED, STARTED/PENDING.
• Public surface additions: IDurableContext.RunInChildContextAsync (typed and void overloads), ChildContextConfig, ChildContextException.
• Service-client mapping extended to copy ContextDetails (Result, partial Error) from SDK responses, plus 14 new unit tests.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Libraries/src/Amazon.Lambda.DurableExecution/IDurableContext.cs	Adds typed/void RunInChildContextAsync to the public interface.
Libraries/src/Amazon.Lambda.DurableExecution/DurableContext.cs	Implements RunInChildContextAsync and constructs ChildContextOperation with a child-context factory.
Libraries/src/Amazon.Lambda.DurableExecution/Internal/ChildContextOperation.cs	New operation type implementing fresh execution, replay, and failure paths.
Libraries/src/Amazon.Lambda.DurableExecution/ChildContextConfig.cs	New config type with SubType and ErrorMapping.
Libraries/src/Amazon.Lambda.DurableExecution/DurableExecutionException.cs	Adds ChildContextException with SubType, ErrorType, ErrorData, OriginalStackTrace.
Libraries/src/Amazon.Lambda.DurableExecution/Services/LambdaDurableServiceClient.cs	Maps ContextDetails from the SDK operation; drops ErrorData/StackTrace.
Libraries/test/Amazon.Lambda.DurableExecution.Tests/ChildContextOperationTests.cs	14 tests covering fresh, replay, suspension, error-mapping, and non-determinism paths.
Libraries/test/Amazon.Lambda.DurableExecution.Tests/LambdaDurableServiceClientTests.cs	New test for ContextDetails Result/Error copy.

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