Allow dismissing the default value to submit an empty input

[shibayan]

Summary

Resolves the long-standing limitation that Prompt.Input<T> with a default value could never return an empty result — Enter on empty input always returned the default (#297).

This adopts the Inquirer-compatible interaction (the de-facto standard across prompt libraries — @inquirer/input prefill: 'tab' and terkelg/prompts both use it):

• Tab inserts the default value into the input buffer for editing
• Backspace with nothing typed dismisses the default value (the hint disappears); pressing Enter afterwards submits an empty value, subject to the usual non-nullable check and validators

No new API is introduced, and the default behavior is fully preserved: Enter on empty input still accepts the default. Both keys previously just beeped in these states, so this is effectively non-breaking.

For reference, Spectre.Console has the same limitation unresolved (DefaultValue short-circuits before AllowEmpty), and the stale external PR #300 proposed the same Tab interaction behind an opt-in flag — this implementation makes the flag unnecessary.

Fixes #297
Closes #300

Test plan

• 4 new interaction tests: Tab inserts default for editing (blue + berry → blueberry), Tab with typed text is a no-op, Backspace dismisses the default (empty submission returns null), dismissed default still enforces required input for non-nullable types
• Full suite: 334 passed, dotnet format clean
• README updated with the key behavior

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Enables an Inquirer-style interaction in Prompt.Input<T>/InputForm<T> so users can edit or dismiss a provided default value, allowing empty submissions when appropriate (notably solving the “can’t return empty if default exists” limitation from #297).

Changes:

• Adds Tab handling to insert the default value into the input buffer for editing.
• Adds Backspace handling on an empty buffer to dismiss the default value.
• Adds interaction tests for the new key behaviors and updates the README to document them.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/Sharprompt.Tests/Forms/FormInteractionTests.cs	Adds interaction tests covering Tab insertion and Backspace dismissal behaviors.
src/Sharprompt/Forms/InputForm.cs	Implements Tab insertion and Backspace dismissal by managing _defaultValue vs. the input buffer.
README.md	Documents the new Tab/Backspace interaction when a default value is present.

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