Add virtual-context support to standalone RunInChildContextAsync

[GarrettBeatty]

Issue

Closes #2461

The .NET Durable Execution SDK exposed virtual (flat) contexts only through the concurrent MapAsync/ParallelAsync paths (NestingType.Flat). There was no equivalent option for a standalone RunInChildContextAsync, so users porting Java/JS examples that manually fan out child contexts and await them with Task.WhenAll had to switch to MapAsync/ParallelAsync.

Investigation

The SDK internals already fully supported virtual standalone child contexts:

• ChildContextOperation<T> takes an isVirtual flag and suppresses its own CONTEXT START/SUCCEED/FAIL checkpoints while still propagating results and exceptions.
• DurableContext.MakeChildFactory handles the virtual case via OperationIdGenerator.CreateVirtualChild, re-parenting inner operations to the nearest non-virtual ancestor.

The only missing piece was public API surface + wiring for the standalone entry point.

Referencing the JS SDK as ground truth, ChildConfig.virtualContext?: boolean drives isVirtual = options?.virtualContext === true in the run-in-child-context handler, which skips checkpoints and re-parents inner ops when virtual.

Change

I chose the NestingType shape (the issue's first suggestion) rather than a virtualContext/IsVirtual bool, because it matches the existing convention on MapConfig and ParallelConfig — consistent for .NET consumers, identical wire behavior to the JS virtualContext bool.

• ChildContextConfig.NestingType — new property, defaults to NestingType.Nested.
• DurableContext.RunChildContext — passes isVirtual: config?.NestingType == NestingType.Flat to the operation.

Users can now write the exact shape requested in the issue:

await context.RunInChildContextAsync(
    async (childContext, ct) =>
        await childContext.StepAsync(
            (_, stepCt) => Task.FromResult("result"),
            name: "compute",
            cancellationToken: ct),
    name: "child-0",
    config: new ChildContextConfig { NestingType = NestingType.Flat });

Tests

Added 3 tests to ChildContextOperationTests:

• RunInChildContextAsync_FlatNesting_EmitsNoContextCheckpointAndReparentsInnerOps — Flat emits no CONTEXT checkpoint; inner ops re-parent to the non-virtual ancestor.
• RunInChildContextAsync_FlatNesting_ReplayReExecutesBodyFromInnerCheckpoint — with no CONTEXT checkpoint, replay re-executes the body while inner steps replay from their own checkpoints (mirrors JS "virtual: re-execute then round-trip").
• RunInChildContextAsync_FlatNesting_FuncThrows_EmitsNoFailCheckpointButPropagates — Flat suppresses the CONTEXT FAIL checkpoint but still propagates the exception.

Build: clean (0 warnings). Tests: 408/408 pass on net8.0 and net10.0.

Change file

Included: .autover/changes/durable-childcontext-virtual.json (Minor, Amazon.Lambda.DurableExecution).