Fix TUI paste burst newline handling

[alexx-ftw]

What changed

• treat Enter/Shift+Enter/Ctrl+Enter as newline during paste bursts before placeholder expansion runs
• keep paste-burst timing window alive after flush so the next newline is still grouped with the paste
• add regression tests for Enter/Shift+Enter/Ctrl+Enter to cover Windows issue [Windows] clipboard paste drops characters, Shift+Enter and Ctrl+Enter insert [13;2u instead of a newline #4401

Why

• Windows consoles without bracketed paste were still submitting the first line of large pastes (issue [Windows] clipboard paste drops characters, Shift+Enter and Ctrl+Enter insert [13;2u instead of a newline #4401)

How to try locally

1. cd codex-rs
2. just fmt
3. just fix -p codex-tui
4. cargo test -p codex-tui -- burst_paste_large

Flags added or removed

• none

Safety limits

• unchanged

Fallback behavior

• existing paste-burst placeholder expansion still applies

Tests run: just fmt, just fix -p codex-tui, cargo test -p codex-tui

[github-actions]

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[alexx-ftw]

I have read the CLA Document and I hereby sign the CLA

[alexx-ftw]

Heads-up: there's a regression in the backtrack flow. Double-tapping Esc in the main chat (composer empty) now crashes with a stack overflow because we re-enter open_backtrack_preview without advancing the selection when the transcript has no user messages left. Logged as issue #4977 for follow-up.

[alexx-ftw]

Updated PR with a fix for #4977: Esc backtrack now reacts only to key presses (ignoring repeats) and closes the overlay when no user messages remain, so double Esc no longer overflows the stack.