Fix durable Map/Parallel replay: MapIteration subtype + inline per-unit results

[GarrettBeatty]

Description

Fixes two durable-execution Map/Parallel conformance issues found by running the multi-language conformance suite against the .NET SDK and comparing with the JS/Python/Java reference SDKs.

1. Map iteration SubType: MapItem → MapIteration

Per-item map iteration child contexts were checkpointed with SubType: "MapItem". The JS, Python, and Java SDKs all use "MapIteration":

• JS: "SubType": "MapIteration"
• Python: MAP_ITERATION = "MapIteration"
• Java: MAP_ITERATION(OperationType.CONTEXT, "MapIteration")

Renamed OperationSubTypes.MapItem → OperationSubTypes.MapIteration and its wire value.

2. Per-unit results lost on post-operation replay (Nested)

A Nested Parallel/Map persisted per-unit results only on the children's own CONTEXT checkpoints, not inline on the parent SUCCEED payload. But the service collapses completed per-unit child contexts out of the state returned on a later resume, so a batch that completes and then suspends (e.g. a wait after the map) reconstructed [null, ...] on replay because the child ops were no longer in state.

Fix: record each dispatched unit's result/error inline on the parent summary for both Nested and Flat nesting (mirroring the JS SDK's default BatchResult serdes, which serializes the whole all array into the parent payload). The reconstruct path prefers the inline copy and falls back to child checkpoints. The overflow-strip + ReplayChildren path now applies to both nesting modes, so large aggregate results still degrade gracefully.

Motivation and Context

Surfaced by cross-SDK conformance testing (map operation 9, 9-17 map-then-wait). Both were real cross-SDK divergences, not test defects.

Testing

• Full Amazon.Lambda.DurableExecution.Tests unit suite: 412/412 pass.
• Added unit tests for both fixes (map + parallel): inline-results-on-parent-payload, and reconstruct-from-inline-summary-without-child-ops (the pruned-resume scenario).
• Validated end-to-end via the multi-language conformance suite against the deployed patched SDK: map 17/17 covered tests pass (9-14/9-19/9-20 uncovered by design — no serdes slots in .NET MapConfig), parallel 21/21 pass (no regression from the shared ConcurrentOperation change).

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.