Add multi-agent runtime metadata types (#25720)

Stack split from #25708. Original PR intentionally left open. This first
PR adds the multi-agent runtime metadata types and catalog plumbing used
by the rest of the stack.

codex-rs/state/src/runtime/threads.rs

@@ -1332,6 +1332,7 @@ mod tests {
                 base_instructions: None,
                 dynamic_tools: None,
                 memory_mode: Some("polluted".to_string()),
+                multi_agent_version: None,
             },
             git: None,
         })];
@@ -1392,6 +1393,7 @@ mod tests {
                 base_instructions: None,
                 dynamic_tools: None,
                 memory_mode: None,
+                multi_agent_version: None,
             },
             git: Some(GitInfo {
                 commit_hash: Some(codex_git_utils::GitSha::new("rollout-sha")),