feat(tui): support Alt-d delete-forward-word

[dougEfresh]

Alt-d should delete the next word. It didn’t. Now it does. Added a small test so it stays that way.

Details:
File updated: codex-rs/tui/src/bottom_pane/textarea.rs
Test added: delete_forward_word_alt_d — verifies Alt-d deletes the next word and keeps the cursor position correct.

Solves Issue #12453

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[dougEfresh]

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

[etraut-openai]

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[dougEfresh]

@etraut-openai Totally understood.

Just saying this is a 4 line change (excluding test) which addresses a mismatch between standard shell key bindings (i.e. readline) and codex TUI.

[etraut-openai]

@dougEfresh, yeah, this is a very contained change. Let me take a closer look at the PR.

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Security review completed. No security issues were found in this pull request.

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

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[etraut-openai]

Even simple keybinding changes like this can be trickier that one might think because of the wide variety of platforms and terminal emulators that we need to support, but this one looks fine to me.

[dougEfresh]

Latest commit uses pretty_assertions::assert_eq and combined the Alt+Delete and Alt+d match arms into a single pattern