feat: forced tool tips (#8752)

Force an announcement tooltip in the CLI. This query the gh repo on this
[file](https://raw.githubusercontent.com/openai/codex/main/announcement_tip.toml)
which contains announcements in TOML looking like this:
```
# Example announcement tips for Codex TUI.
# Each [[announcements]] entry is evaluated in order; the last matching one is shown.
# Dates are UTC, formatted as YYYY-MM-DD. The from_date is inclusive and the to_date is exclusive.
# version_regex matches against the CLI version (env!("CARGO_PKG_VERSION")); omit to apply to all versions.
# target_app specify which app should display the announcement (cli, vsce, ...).

[[announcements]]
content = "Welcome to Codex! Check out the new onboarding flow."
from_date = "2024-10-01"
to_date = "2024-10-15"
version_regex = "^0\\.0\\.0$"
target_app = "cli"
``` 

To make this efficient, the announcement is queried on a best effort
basis at the launch of the CLI (no refresh made after this).
This is done in an async way and we display the announcement (with 100%
probability) iff the announcement is available, the cache is correctly
warmed and there is a matching announcement (matching is recomputed for
each new session).

codex-rs/tui/src/lib.rs

@@ -97,7 +97,6 @@ pub use markdown_render::render_markdown_text;
 pub use public_widgets::composer_input::ComposerAction;
 pub use public_widgets::composer_input::ComposerInput;
 use std::io::Write as _;
-
 // (tests access modules directly within the crate)
 
 pub async fn run_main(
@@ -344,6 +343,8 @@ async fn run_ratatui_app(
 ) -> color_eyre::Result<AppExitInfo> {
     color_eyre::install()?;
 
+    tooltips::announcement::prewarm();
+
     // Forward panic reports through tracing so they appear in the UI status
     // line, but do not swallow the default/color-eyre panic handler.
     // Chain to the previous hook so users still get a rich panic report