chore: rename collab feature flag key to multi_agent (#11918)

Summary
- rename the collab feature key to multi_agent while keeping the Feature
enum unchanged
- add legacy alias support so both "multi_agent" and "collab" map to the
same feature
- cover the alias behavior with a new unit test

codex-rs/core/src/features/legacy.rs

@@ -33,6 +33,10 @@ const ALIASES: &[Alias] = &[
         legacy_key: "web_search",
         feature: Feature::WebSearchRequest,
     },
+    Alias {
+        legacy_key: "collab",
+        feature: Feature::Collab,
+    },
 ];
 
 pub(crate) fn legacy_feature_keys() -> impl Iterator<Item = &'static str> {