refactor TUI event loop to enable dropping + recreating crossterm event stream (#7961)

Introduces an `EventBroker` between the crossterm `EventStream` source
and the consumers in the TUI. This enables dropping + recreating the
`crossterm_events` without invalidating the consumer.

Dropping and recreating the crossterm event stream enables us to fully
relinquish `stdin` while the app keeps running. If the stream is not
dropped, it will continue to read from `stdin` even when it is not
actively being polled, potentially stealing input from other processes.
See
[here](https://www.reddit.com/r/rust/comments/1f3o33u/myterious_crossterm_input_after_running_vim/?utm_source=chatgpt.com)
and [here](https://ratatui.rs/recipes/apps/spawn-vim/) for details.

### Tests
Added tests for new `EventBroker` setup, existing tests pass, tested
locally.

codex-rs/Cargo.lock

@@ -1701,7 +1701,6 @@ dependencies = [
  "anyhow",
  "arboard",
  "assert_matches",
- "async-stream",
  "base64",
  "chrono",
  "clap",
@@ -6910,6 +6909,7 @@ dependencies = [
  "futures-core",
  "pin-project-lite",
  "tokio",
+ "tokio-util",
 ]
 
 [[package]]