fix(tui): restore alt-enter newline alias

[fcoury-oai]

Fixes #20501
Refs #20580.

Summary

• add Alt+Enter to the built-in editor newline aliases
• update keymap tests that used Alt+Enter as a custom submit binding now that it conflicts with newline
• refresh the keymap action-menu snapshot fixture

Test Plan

• just fmt
• cargo test -p codex-tui keymap::tests
• cargo test -p codex-tui bottom_pane::textarea::tests
• cargo test -p codex-tui keymap_setup::tests
• cargo test -p codex-tui
• cargo insta pending-snapshots
• git diff --check
• just argument-comment-lint