Add IAsyncEnumerable support to task computation expressions

[Copilot]

Task-based computation expressions (taskResult, taskOption, taskValidation, taskValueOption, and their backgroundTask* variants) did not support for .. in .. iteration over IAsyncEnumerable<T> sequences.

Changes

Core builders (TaskResultCE.fs, TaskOptionCE.fs, TaskValidationCE.fs, TaskValueOptionCE.fs)

• WhileAsync — async condition loop that awaits ValueTask<bool> before each iteration; built on this.While (not ResumableCode.While) so error/None short-circuit stops the loop without an extra MoveNextAsync() call. Implements both the static state machine (__useResumableCode) and dynamic paths.
• For(#IAsyncEnumerable<'T>) — uses this.Using for async disposal of the enumerator + WhileAsync with e.MoveNextAsync().
• Source(#IAsyncEnumerable<_>) — added in a separate [<AutoOpen>] extension module to avoid CLR signature conflicts with the existing Source(#seq<_>) method (both compile to the same generic signature on the same type).

backgroundTask* builders inherit from these base classes and gain support automatically.

Tests

• TestHelpers.toAsyncEnumerable — minimal IAsyncEnumerable<T> wrapper over a list for test use.
• Happy-path and early-exit (error/None) tests added for all seven affected CE builders.

Example

let items : IAsyncEnumerable<string> = getItemsAsync()

let! result =
    taskResult {
        for item in items do
            let! validated = validate item
            do! save validated
        return "done"
    }
// Stops iteration immediately on the first Error

Original prompt

---

This section details on the original issue you should resolve

<issue_title>IAsyncEnumerable</issue_title>
<issue_description>I noticed FSharp.Control.TaskSeq isn't supported with computation expressions like taskResult, any plans?
</issue_description>

<agent_instructions>- Make sure you've added tests and they pass

• Make sure you've added documentation and doc comments
• Make sure you've researched the implementation in IcedTasks</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@TheAngryByrd 👋 While I wouldn't depend on `FSharp.Control.TaskSeq` for this, I'd accept a PR for handling `IAsyncEnumerable`. Here's an example of the [implementation in IcedTasks](https://github.com/TheAngryByrd/IcedTasks/blob/db0b55bd89f769e17744319bfc510486a5b1fc5f/src/IcedTasks/TaskBuilderBase.fs#L343-L357) someone could use for reference.

• Fixes IAsyncEnumerable #275

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[Copilot]

Pull request overview

This PR adds IAsyncEnumerable<T> support to task-based computation expressions, enabling for .. in .. iteration over async sequences with early exit on error/None conditions.

Changes:

• Implemented WhileAsync and For(#IAsyncEnumerable<'T>) methods in core builder base classes
• Added Source(#IAsyncEnumerable<_>) extension methods in separate auto-open modules to avoid CLR signature conflicts
• Added test helper toAsyncEnumerable and comprehensive tests for all affected builders

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/FsToolkit.ErrorHandling.Tests/TestHelpers.fs	Added toAsyncEnumerable helper to create test IAsyncEnumerable<T> instances from lists
tests/FsToolkit.ErrorHandling.Tests/TaskValueOptionCE.fs	Added tests for IAsyncEnumerable iteration in taskValueOption with happy path and early exit scenarios
tests/FsToolkit.ErrorHandling.Tests/TaskValidationCE.fs	Added tests for IAsyncEnumerable iteration in taskValidation with happy path and early exit scenarios
tests/FsToolkit.ErrorHandling.Tests/TaskResultCE.fs	Added tests for IAsyncEnumerable iteration in taskResult with happy path and early exit scenarios
tests/FsToolkit.ErrorHandling.Tests/TaskOptionCE.fs	Added tests for IAsyncEnumerable iteration in taskOption with happy path and early exit scenarios
tests/FsToolkit.ErrorHandling.Tests/BackgroundTaskValueOptionCE.fs	Added tests for IAsyncEnumerable iteration in backgroundTaskValueOption with happy path and early exit scenarios
tests/FsToolkit.ErrorHandling.Tests/BackgroundTaskResultCE.fs	Added tests for IAsyncEnumerable iteration in backgroundTaskResult with happy path and early exit scenarios
tests/FsToolkit.ErrorHandling.Tests/BackgroundTaskOptionCE.fs	Added tests for IAsyncEnumerable iteration in backgroundTaskOption with happy path and early exit scenarios
src/FsToolkit.ErrorHandling/TaskValueOptionCE.fs	Implemented WhileAsync, For(#IAsyncEnumerable<'T>), and Source extension for TaskValueOptionBuilderBase
src/FsToolkit.ErrorHandling/TaskValidationCE.fs	Implemented WhileAsync, For(#IAsyncEnumerable<'T>), and Source extension for TaskValidationBuilderBase
src/FsToolkit.ErrorHandling/TaskResultCE.fs	Implemented WhileAsync, For(#IAsyncEnumerable<'T>), and Source extension for TaskResultBuilderBase
src/FsToolkit.ErrorHandling/TaskOptionCE.fs	Implemented WhileAsync, For(#IAsyncEnumerable<'T>), and Source extension for TaskOptionBuilderBase

[TheAngryByrd]

@copilot open a new PR and create gitbook docs to show off this new feature