Source proactive multi-agent instructions from the model catalog (#41457)

## What changed

- Add an optional `proactive` multi-agent mode message to model metadata.
- Use the catalog's proactive message for `Ultra` reasoning when no general mode hint is configured. A missing value falls back to the built-in proactive instructions, while an empty value suppresses the mode message.
- Keep explicit mode behavior for other reasoning efforts and refresh the applicable catalog message when the model changes.

## Testing

- Cover proactive overrides, built-in fallback, empty-value suppression, mode-hint precedence, non-`Ultra` behavior, and model switches.

GitOrigin-RevId: da0a9ebd9b58cf04ef13a703c210e5da2eed0884

codex-rs/core/src/session/multi_agents.rs

@@ -165,7 +165,10 @@ pub(crate) fn effective_multi_agent_mode(turn_context: &TurnContext) -> Option<M
     let multi_agent_mode = match mode_hint_text {
         Some(hint_text) => MultiAgentMode::Custom(hint_text.to_string()),
         None => match turn_context.effective_reasoning_effort() {
-            Some(ReasoningEffort::Ultra) => MultiAgentMode::Proactive,
+            Some(ReasoningEffort::Ultra) => catalog_mode
+                .and_then(|messages| messages.proactive.clone())
+                .map(MultiAgentMode::Custom)
+                .unwrap_or(MultiAgentMode::Proactive),
             _ => catalog_mode
                 .and_then(|messages| messages.explicit.clone())
                 .map(MultiAgentMode::Custom)