[Context]: add MapInContextAsync collection transform

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Summary

Add MapInContextAsync — an async LINQ-like transform of IEnumerable<TIn> → results, where each item's transform runs inside a safe context scope.

Goal

Provide a clean, readable alternative to a hand-written Select(...) + Task.WhenAll loop that loses/leaks ambient context.

Problem

The current parallel primitives are single-action or all-delegates. Transforming a collection item-by-item with proper context isolation is boilerplate and error-prone:

// ❌ manual
var tasks = items.Select(i => manager.ExecuteInContext(ctx, () => Transform(i)));
var results = await Task.WhenAll(tasks);

Scope

• IContextManager<C>.MapInContextAsync(C ctx, IEnumerable<TIn>, Func<TIn,Task<TOut>>).
• Each item transformed under its own context snapshot (isolation, no ambient write).
• Result order preserved by default (see the Ordered/Unordered issue for opt-out).
• Unit tests for context flow, isolation, and order.

Design Expectations

• Composes with the base fork-join execution primitive.
• Reuses per-item BeginContext/snapshot; no new runtime primitive.
• Async-only; ConfigureAwait(false) internally.

Acceptance Criteria

• Collection transforms with correct per-item context isolation.
• Order of results matches input order by default.
• No behavior change for a single-item input.

Non-Goals

• No concurrency limiting (lives in MapParallel/MaxConcurrency).
• No partial-success handling (lives in TryEach).
• No chunking.