Case insensitive user text input comparison

[akinsmosu]

fixes #1906

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Changes

Summary
This PR makes TextPrompt<string> case-insensitive when matching user input against defined choices. Previously, inputs such as "yes" would fail to match "Yes".

What this change does
When the TextPrompt's generic is a string and the user did not explicitly provide a comparer, it automatically configures _comparer to StringComparer.OrdinalIgnoreCase.

Adds a theory-based test to verify case-insensitive matching.

All existing behavior remains unchanged for non-string TextPrompt<T> usage.

Implementation Notes
Added a typeof(T) == typeof(string) check in TextPrompt<T> to detect string-based prompts. _comparer is set to StringComparison.OrdinalIgnoreCase only if the user does not supply a comparer.

[akinsmosu]

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[patriksvensson]

Great first PR, but there are some changes I would like to see before I merge this 👍

[akinsmosu]

Thanks for the feedback! All suggestions applied.