feat(tui): add /exit slash command (#5932)

[zemaj]

Summary

• promote /exit to a first-class slash command (aliasing /quit) and keep dispatch wiring intact
• normalize the parser so /exit preserves its spelling while still accepting /quit
• add parser + ChatWidget harness coverage and document /exit in docs/slash-commands.md

Testing

• ./build-fast.sh
• cargo test -p code-tui slash_exit_and_quit_dispatch_exit_command (fails: local cargo registry copy of cc 1.2.41 is missing generated modules; clear/update the crate and rerun)

Closes openai#5932.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Make drain_events public without exporting AppEvent

Changing ChatWidgetHarness::drain_events to pub exposes a return type of Vec<AppEvent> while AppEvent itself remains pub(crate). When the crate is built with the test-helpers feature (required for the newly added integration test), ChatWidgetHarness is re‑exported and this method becomes part of the public API, causing error[E0446]: private type AppEvent in public interface and the crate no longer compiles. Either keep the method crate‑visible or publicly re‑export AppEvent so the public signature uses a public type.

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[zemaj]

✅ Reviewed locally in an isolated worktree.

Key notes:

• Tightened process_slash_command_message so /exit (and /quit) only trigger with a leading slash, reject extra args, and normalise casing—bare words now fall back to regular chat text.
• Guarded SlashCommand::Exit inside submit_user_message so synthetic/suppressed submissions (Auto Drive, CLI reroutes) can no longer exit the app, and skipped history persistence/dispatch in that path.
• Updated the App dispatcher to avoid adding /exit to the session history; added harness tests covering both suppressed and normal /exit flows plus parser edge cases.
• Build + targeted unit tests: cargo test -p code-tui --lib slash_exit_and_quit_dispatch_exit_command, cargo test -p code-tui --lib exit_command_ignored_for_suppressed_message, cargo test -p code-tui --lib exit_command_dispatches_without_history_persistence, and ./build-fast.sh.

Follow-up: full app-level integration coverage of the exit path is still missing; if UX ever needs confirmation/cleanup semantics, it should be added alongside an integration test.