Suppress empty multi-agent mode messages (#33862)

## What changed

- Treat an empty custom `multi_agent_mode_hint_text` as having no instructions to emit.
- Avoid adding an empty multi-agent mode message both on the initial turn and when the configured hint changes after resuming a session.

## Testing

- Update the empty-hint integration test to assert that no mode message is sent.
- Add resume coverage that verifies changing a custom hint to an empty string emits no update.

GitOrigin-RevId: 8033292115936fac2cf22f85141278595804ee61

codex-rs/core/src/context_manager/updates.rs

@@ -95,9 +95,11 @@ fn build_multi_agent_mode_update_item(
     }
 
     match effective_multi_agent_mode {
-        Some(multi_agent_mode) => Some(MultiAgentModeInstructions::new(multi_agent_mode).render()),
+        Some(multi_agent_mode) => MultiAgentModeInstructions::from_mode(multi_agent_mode)
+            .map(|instructions| instructions.render()),
         None if previous.multi_agent_mode == Some(MultiAgentMode::Proactive) => {
-            Some(MultiAgentModeInstructions::new(MultiAgentMode::ExplicitRequestOnly).render())
+            MultiAgentModeInstructions::from_mode(MultiAgentMode::ExplicitRequestOnly)
+                .map(|instructions| instructions.render())
         }
         None => None,
     }