Rename collab modules to multi agents (#11939)

Summary
- rename the `collab` handlers and UI files to `multi_agents` to match
the new naming
- update module references and specs so the handlers and TUI widgets
consistently use the renamed files
- keep the existing functionality while aligning file and module names
with the multi-agent terminology

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

@@ -1,11 +1,11 @@
 pub mod apply_patch;
-pub(crate) mod collab;
 mod dynamic;
 mod grep_files;
 mod js_repl;
 mod list_dir;
 mod mcp;
 mod mcp_resource;
+pub(crate) mod multi_agents;
 mod plan;
 mod read_file;
 mod request_user_input;
@@ -20,7 +20,6 @@ use serde::Deserialize;
 
 use crate::function_tool::FunctionCallError;
 pub use apply_patch::ApplyPatchHandler;
-pub use collab::CollabHandler;
 pub use dynamic::DynamicToolHandler;
 pub use grep_files::GrepFilesHandler;
 pub use js_repl::JsReplHandler;
@@ -28,6 +27,7 @@ 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_user_input::RequestUserInputHandler;