Show AWS GovCloud guidance at startup and remember acknowledgment (#52176)

## Why

GovCloud guidance was tied to Bedrock credential setup, so users with an already configured provider could miss it.

## What changed

Check for GovCloud during TUI startup using the app server's resolved provider, including when connecting to a server. Require acknowledgment before proceeding and save `notice.hide_gov_cloud_guidance = true` to suppress the screen on subsequent launches. Quitting the guidance exits startup without saving acknowledgment.

## Testing

Add a terminal integration test covering acknowledgment persistence, quitting without saving, and suppression after restart. Retain coverage for explicit acknowledgment and scrolling in short panes.

GitOrigin-RevId: f09fd73a66fb02f94ebdd3c3b0dbe5712dfa9be7

codex-rs/config/src/types.rs

@@ -1007,6 +1007,8 @@ pub struct ExternalConfigMigrationPrompts {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema)]
 #[schemars(deny_unknown_fields)]
 pub struct Notice {
+    /// Tracks whether the user has acknowledged the AWS GovCloud guidance.
+    pub hide_gov_cloud_guidance: Option<bool>,
     /// Tracks whether the user has acknowledged the full access warning prompt.
     pub hide_full_access_warning: Option<bool>,
     /// Tracks whether the user has acknowledged the Windows world-writable directories warning.