fixes #

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• I have commented on the issue above and discussed the intended changes my proposal for #595
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

This PR allows the insertion of the default value inside text field/input buffer making it editable and already available instead of the current DefaultValue behavior to simply set the text variable with the default when empty.

1. TextPrompt.cs: Added a bool DefaultInput { get; set; } property and, when showing the prompt, if enabled the default value is placed into the editable input field.

2. TextPromptExtensions.cs: Added matching extension DefaultInput<T>(this TextPrompt<T> obj, bool state) to enable/disable the editable default behavior when creating TextPrompts.

3. AnsiConsoleExtensions.Input.cs: Added a new ReadLine() overload that accepts an initialInput (string?) and enqueues a ConsoleKeyInfo per character. The overload consumes those queued key events first, then falls back to console.Input.ReadKeyAsync().

4. TextPromptTests.cs: Added a unit test Should_Return_Error_If_Default_Choice_Invalid that checks the default input system (Testing the validity checks whether the input matches a valid choice in the list).

Affected files:

• src/Spectre.Console/Prompts/TextPrompt.cs
• src/Spectre.Console/Prompts/TextPromptExtensions.cs
• src/Spectre.Console/Extensions/AnsiConsoleExtensions.Input.cs
• src/Spectre.Console.Tests/Unit/Prompts/TextPromptTests.cs
• src/Spectre.Console.Tests/Expectations/Prompts/Text/InvalidDefaultChoice.Output.verified.txt

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