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/function_tool.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Debug, Error, PartialEq)]
+pub enum FunctionCallError {
+    #[error("{0}")]
+    RespondToModel(String),
+}