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;

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

@@ -21,7 +21,6 @@ use crate::exec_command::exec_command_params::WriteStdinParams;
 use crate::exec_command::exec_command_session::ExecCommandSession;
 use crate::exec_command::session_id::SessionId;
 use crate::truncate::truncate_middle;
-use codex_protocol::models::FunctionCallOutputPayload;
 
 #[derive(Debug, Default)]
 pub struct SessionManager {
@@ -38,7 +37,7 @@ pub struct ExecCommandOutput {
 }
 
 impl ExecCommandOutput {
-    fn to_text_output(&self) -> String {
+    pub(crate) fn to_text_output(&self) -> String {
         let wall_time_secs = self.wall_time.as_secs_f32();
         let termination_status = match self.exit_status {
             ExitStatus::Exited(code) => format!("Process exited with code {code}"),
@@ -68,19 +67,6 @@ pub enum ExitStatus {
     Ongoing(SessionId),
 }
 
-pub fn result_into_payload(result: Result<ExecCommandOutput, String>) -> FunctionCallOutputPayload {
-    match result {
-        Ok(output) => FunctionCallOutputPayload {
-            content: output.to_text_output(),
-            success: Some(true),
-        },
-        Err(err) => FunctionCallOutputPayload {
-            content: err,
-            success: Some(false),
-        },
-    }
-}
-
 impl SessionManager {
     /// Processes the request and is required to send a response via `outgoing`.
     pub async fn handle_exec_command_request(