fix(cli): restore paused stdin after capability detection

[sylvesterkaczmarek]

Fixes #28799

Summary

• restore stdin to paused mode after terminal capability detection when it was not already flowing
• preserve an already-flowing stdin stream
• add focused regression coverage for both flow-state cases

Why

detectCapabilities() temporarily attaches a data listener to process.stdin. In Node.js, adding a data listener switches a readable stream into flowing mode, and removing that listener does not automatically pause the stream again. With no consumer attached, subsequent input can be discarded before Ink installs its readable consumer.

This matches #28799's observed state: terminal capability detection completes, the TUI renders, but the tty is never subsequently consumed and keyboard input is lost.

The cleanup now restores the pre-detection non-flowing state by calling pause() only when stdin was not already flowing.

Validation

Added focused tests in packages/cli/src/ui/utils/terminalCapabilityManager.stdin-flow.test.ts covering:

• non-flowing stdin is paused after detection
• already-flowing stdin is preserved

[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 an issue where terminal capability detection inadvertently leaves stdin in a flowing mode, causing subsequent keyboard input to be lost. By tracking the stream's state before detection and conditionally calling pause, the fix ensures that the terminal remains responsive for subsequent consumers like Ink.

Highlights

• Stdin Flow State Management: Updated terminal capability detection to track the initial flow state of stdin, ensuring it is restored to a paused state only if it was not already flowing.
• Regression Testing: Added a new test suite to verify that stdin is correctly paused after detection when idle, and remains in flowing mode if it was already active.

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[github-actions]

📊 PR Size: size/M

• Lines changed: 105
• Additions: +105
• Deletions: -0
• Files changed: 2

[gemini-code-assist]

Code Review

This pull request ensures that process.stdin is paused after temporary terminal capability detection if it was not already flowing. This prevents keyboard input from being discarded before Ink attaches its consumer. A new test file has been added to verify this behavior. I have no feedback to provide as the implementation is correct and there are no review comments.