[codex] Dedupe fallback model metadata warnings (#21090)

Fixes #21070.

This is a small cleanup around model metadata handling for
gateway/provider model names. It follows the report and proposed
direction from @dkbush by keeping the fallback metadata warning useful
without repeating it every turn, and by tightening the existing
provider-prefix lookup path.

- Track fallback metadata warning slugs in session state so each
unresolved model warns once per session.
- Keep warning emission outside the session-state lock and preserve the
existing warning text.
- Allow one-segment provider prefixes with hyphenated provider IDs,
while preserving the multi-segment rejection behavior.
- Add focused coverage for warning dedupe and hyphenated provider-prefix
metadata matching.

Testing:

- Ran `just fmt`.
- Ran `git diff --check`.
- Added tests for the new warning dedupe and provider-prefix lookup
behavior.

codex-rs/tui/src/chatwidget.rs

@@ -351,6 +351,8 @@ use self::status_surfaces::TerminalTitleStatusKind;
 mod user_messages;
 use self::user_messages::PendingSteerCompareKey;
 use self::user_messages::UserMessageDisplay;
+mod warnings;
+use self::warnings::WarningDisplayState;
 pub(crate) use crate::branch_summary::StatusLineGitSummary;
 use crate::streaming::chunking::AdaptiveChunkingPolicy;
 use crate::streaming::commit_tick::CommitTickScope;
@@ -780,6 +782,7 @@ pub(crate) struct ChatWidget {
     plan_type: Option<PlanType>,
     codex_rate_limit_reached_type: Option<RateLimitReachedType>,
     rate_limit_warnings: RateLimitWarningState,
+    warning_display_state: WarningDisplayState,
     rate_limit_switch_prompt: RateLimitSwitchPromptState,
     add_credits_nudge_email_in_flight: Option<AddCreditsNudgeCreditType>,
     adaptive_chunking: AdaptiveChunkingPolicy,
@@ -3133,7 +3136,11 @@ impl ChatWidget {
     }
 
     fn on_warning(&mut self, message: impl Into<String>) {
-        self.add_to_history(history_cell::new_warning_event(message.into()));
+        let message = message.into();
+        if !self.warning_display_state.should_display(&message) {
+            return;
+        }
+        self.add_to_history(history_cell::new_warning_event(message));
         self.request_redraw();
     }
 
@@ -4941,6 +4948,7 @@ impl ChatWidget {
             plan_type: initial_plan_type,
             codex_rate_limit_reached_type: None,
             rate_limit_warnings: RateLimitWarningState::default(),
+            warning_display_state: WarningDisplayState::default(),
             rate_limit_switch_prompt: RateLimitSwitchPromptState::default(),
             add_credits_nudge_email_in_flight: None,
             adaptive_chunking: AdaptiveChunkingPolicy::default(),