fix(cli): restrict ctrl+backspace detection to Windows Terminal only

[dogukanozen]

Summary

• PR fix(cli-ui): enable Ctrl+Backspace for word deletion in Windows Terminal #21447 introduced ctrl+backspace word deletion for Windows, but the condition OS === 'Windows_NT' was too broad
• cmd.exe and PowerShell send \b (0x08) for plain backspace, not \x7f — so they were incorrectly being treated as Ctrl+Backspace, deleting a whole word on every backspace press
• Fixed by scoping ctrl=true to WT_SESSION only (Windows Terminal), which is the only terminal that sends \b for Ctrl+Backspace and \x7f for plain backspace
• Updated the test that expected OS=Windows_NT alone to set ctrl=true

Fixes #25867

Test plan

• Windows Terminal (WT_SESSION set): Ctrl+Backspace deletes word, plain backspace deletes one character
• cmd.exe / PowerShell (no WT_SESSION): plain backspace deletes one character (regression from fix(cli-ui): enable Ctrl+Backspace for word deletion in Windows Terminal #21447 is fixed)
• Linux/macOS: behavior unchanged

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a regression in input handling for Windows users. By narrowing the scope of Ctrl+Backspace detection to specifically target Windows Terminal sessions, the change ensures that standard terminal emulators like cmd.exe and PowerShell maintain correct backspace behavior, preventing accidental word deletion.

Highlights

• Refined Ctrl+Backspace Detection: Restricted the detection of Ctrl+Backspace to only occur when the Windows Terminal environment variable (WT_SESSION) is present.
• Regression Fix: Prevented standard backspace keys in cmd.exe and PowerShell from being incorrectly interpreted as Ctrl+Backspace, which previously caused unintended word deletions.
• Test Suite Update: Updated the KeypressContext test suite to verify that backspace is not treated as a control key when running on Windows without an active Windows Terminal session.

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[gemini-code-assist]

Code Review

This pull request refines the handling of the backspace character (\b) in Windows environments within the KeypressContext. The logic now specifically checks for the WT_SESSION environment variable to identify Windows Terminal, ensuring that \b is only treated as Ctrl+Backspace in that context, while standard consoles like cmd.exe and PowerShell treat it as a regular backspace. The associated test case has been updated to verify this behavior. I have no feedback to provide.