feat(tui): improve TUI keymap coverage

[fcoury-oai]

Summary

• normalize terminal-emitted C0 control characters through configurable editor keymaps, covering raw control-key fallbacks like Shift+Enter-as-LF in terminals from Shift + Enter newline ignored in Zed integrated terminal #20555 and Zed Shift+Enter sends LF and bypasses Ctrl+J newline binding #20898, plus part of the modified-Enter behavior in Regression: Shift+Enter / Alt+Enter no longer insert newline in TUI prompt #20580
• add default-unbound keymap actions for toggling Fast mode and killing the current composer line, giving TUI: support zsh-style Ctrl+U as a configurable line-kill command #20698 users a configurable zsh-style Ctrl+U option without changing the existing default Ctrl+U behavior
• wire the new actions through gated /keymap picker entries, schema generation, and snapshot coverage

Fixes #20555.
Fixes #20898.

Testing

• just write-config-schema
• just fmt
• cargo test -p codex-config
• 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 insta pending-snapshots
• just fix -p codex-tui
• git diff --check
• just argument-comment-lint