Split multi-agent handlers per tool (#14535)

Summary
- move the existing multi-agent handler logic into each tool-specific
handler and inline helper implementations
- remove the old central dispatcher now that each handler encapsulates
its own behavior
- adjust handler specs and tests to match the new structure without
macros

Testing
- Not run (not requested)

codex-rs/core/src/tools/handlers/mod.rs

@@ -44,7 +44,6 @@ pub use js_repl::JsReplResetHandler;
 pub use list_dir::ListDirHandler;
 pub use mcp::McpHandler;
 pub use mcp_resource::McpResourceHandler;
-pub use multi_agents::MultiAgentHandler;
 pub use plan::PlanHandler;
 pub use read_file::ReadFileHandler;
 pub use request_permissions::RequestPermissionsHandler;