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/config.schema.json

@@ -233,6 +233,9 @@
             "memory_tool": {
               "type": "boolean"
             },
+            "multi_agent": {
+              "type": "boolean"
+            },
             "personality": {
               "type": "boolean"
             },
@@ -1363,6 +1366,9 @@
         "memory_tool": {
           "type": "boolean"
         },
+        "multi_agent": {
+          "type": "boolean"
+        },
         "personality": {
           "type": "boolean"
         },