Simplify tool implemetations (#4160)

Use Result<String, FunctionCallError> for all tool handling code and
rely on error propagation instead of creating failed items everywhere.

codex-rs/core/src/exec_command/mod.rs

@@ -12,4 +12,3 @@ pub use responses_api::WRITE_STDIN_TOOL_NAME;
 pub use responses_api::create_exec_command_tool_for_responses_api;
 pub use responses_api::create_write_stdin_tool_for_responses_api;
 pub use session_manager::SessionManager as ExecSessionManager;
-pub use session_manager::result_into_payload;