[mkarimdev]

Summary

Adds opt-in free-form input to Prompt.Select<T>, so a pick list can also accept a value that is not in the list. Fixes #336.

var fruit = Prompt.Select(options =>
{
    options.Message = "Fruit";
    options.Items = ["apple", "banana"];

    // Only called when Enter cannot resolve a listed item.
    options.CreateCustomItem = text => text.Trim();
});

Behavior

Resolution order on Enter is unchanged except for the new fallback:

1. An explicitly highlighted item (↑/↓) always wins.
2. An exact, case-insensitive text match wins.
3. Otherwise, with CreateCustomItem set and non-empty typed text, the callback produces the result.
4. Otherwise the existing validation error is shown.

Details:

• CreateCustomItem defaults to null; when it is not set, every existing outcome, error message, and single-item auto-selection is preserved.
• A single filtered item is not auto-selected while a callback is configured and text is typed. This lets a user type app and create it even when apple is the only match; pressing ↓ still selects apple. The same policy drives rendering, so the highlighted row and the Enter result cannot disagree.
• Empty Items is accepted only when the callback is set. Without a callback it still throws ArgumentException.
• A callback exception is shown as a prompt error and the prompt continues. A null return is rejected with Validation_Invalid.
• The callback receives the raw typed text (no trim/case folding); the answer is rendered through TextSelector like any other item.
• No reflection or TypeConverter use, so the feature is Native AOT safe.

MultiSelect is unchanged: the public Paginator.TryGetSelectedItem signature is retained and the new overload is internal.

API

• SelectOptions<T>.CreateCustomItem (Func<string, T>?)
• SelectOptionsExtensions.WithCreateCustomItem(...)

Tests

18 new tests:

• PaginatorTests: auto-select on/off, exact match, explicit highlight, empty list.
• FormInteractionTests: creation, exact-match and highlight precedence, partial match without a false highlight, empty filter, empty Items with/without callback, factory exception, null return, no-callback regression, custom TextSelector.
• FluentExtensionsTests: option assignment and null-argument validation.

Sharprompt.Tests (484) and Sharprompt.SourceGenerator.Tests (210) pass in Release; dotnet format reports no changes.