* Return Pending when a checkpoint response has no checkpoint token, instead of failing the execution.

* Treat the service's 'Invalid checkpoint token' error as transient (case-insensitive) so a stale token retries the invocation instead of failing the execution.

* Add SubType to StepConfig and CallbackConfig, and validate SubType values (1 to 32 characters from [a-zA-Z0-9-_]) including ChildContextConfig.SubType.

* Add TestRunnerOptions.OmitCheckpointToken to simulate a checkpoint response without a token.

* Add ILambdaContext.W3C to expose the W3C trace-context fields (traceparent, tracestate, baggage) carried on clientContext.w3c at invoke time.

* Implement ILambdaContext.W3C in LambdaContext, surfacing the allowlisted W3C trace-context fields (traceparent, tracestate, baggage) parsed from the client context header.

* Add W3C trace-context support to TestLambdaContext via the settable W3CFields property and the W3C property.

* Implement ILambdaContext.W3C on the SnapStart empty context, returning an empty trace-context during initialization.