open_ai: Fix error message not showing up when using ChatGPT subscription (#57750)

The API seems to return nested errors, so made the error deserialise
properly in case we get `{ "error": {...} }` instead of a top-level
error

Closes #57024

For testing, you can prompt something like: `tell me about
https://registry.npmjs.org/vite-plus.`

Before:

<img width="631" height="69" alt="image"
src="https://github.com/user-attachments/assets/5d02e7ec-8176-4bff-87d7-908ac8f0b498"
/>

After:

<img width="697" height="61" alt="image"
src="https://github.com/user-attachments/assets/97fac249-8b76-463c-8483-a150f5db9857"
/>


Release Notes:

- openai: Fixed an issue where error messages would not show up properly

crates/open_ai/src/completion.rs

@@ -881,8 +881,13 @@ impl OpenAiResponseEventMapper {
                 let message = response_failure_message(&response);
                 vec![Err(LanguageModelCompletionError::Other(anyhow!(message)))]
             }
-            ResponsesStreamEvent::Error { error }
-            | ResponsesStreamEvent::GenericError { error } => {
+            ResponsesStreamEvent::Error { error } => {
+                vec![Err(LanguageModelCompletionError::Other(anyhow!(
+                    response_error_message(&error)
+                )))]
+            }
+            ResponsesStreamEvent::GenericError { error } => {
+                let error = error.into_response_error();
                 vec![Err(LanguageModelCompletionError::Other(anyhow!(
                     response_error_message(&error)
                 )))]
@@ -2214,6 +2219,34 @@ mod tests {
         assert_eq!(error.to_string(), "ERR_SOMETHING: Something went wrong");
     }
 
+    #[test]
+    fn responses_stream_deserializes_nested_error_event() {
+        // In practice the Responses API often nests error fields under an
+        // `error` object even though the public spec documents them at the top
+        // level. Make sure we don't lose the message and code in that case.
+        let event = serde_json::from_value::<ResponsesStreamEvent>(json!({
+            "type": "error",
+            "error": {
+                "type": "invalid_request_error",
+                "code": "context_length_exceeded",
+                "message": "Your input exceeds the context window of this model. Please adjust your input and try again.",
+                "param": "input"
+            },
+            "sequence_number": 2
+        }))
+        .expect("nested error event");
+
+        let mut mapper = OpenAiResponseEventMapper::new();
+        let mapped = mapper.map_event(event);
+
+        assert_eq!(mapped.len(), 1);
+        let error = mapped.into_iter().next().unwrap().unwrap_err();
+        assert_eq!(
+            error.to_string(),
+            "context_length_exceeded: Your input exceeds the context window of this model. Please adjust your input and try again."
+        );
+    }
+
     #[test]
     fn responses_stream_deserializes_response_error_event() {
         let event = serde_json::from_value::<ResponsesStreamEvent>(json!({

crates/open_ai/src/responses.rs

@@ -158,6 +158,43 @@ pub struct ResponseError {
     pub param: Option<Value>,
 }
 
+/// Payload of the top-level `error` SSE event from the Responses API.
+///
+/// OpenAI's spec documents the error fields as being at the top level of the
+/// event, but in practice the API often nests them under an `error` object.
+#[derive(Deserialize, Debug, Clone, Default)]
+pub struct GenericStreamErrorPayload {
+    #[serde(flatten)]
+    top_level: PartialResponseError,
+    #[serde(default)]
+    error: Option<PartialResponseError>,
+}
+
+#[derive(Deserialize, Debug, Clone, Default)]
+struct PartialResponseError {
+    #[serde(default)]
+    code: Option<String>,
+    #[serde(default)]
+    message: Option<String>,
+    #[serde(default)]
+    param: Option<Value>,
+}
+
+impl GenericStreamErrorPayload {
+    pub fn into_response_error(self) -> ResponseError {
+        let nested = self.error.unwrap_or_default();
+        ResponseError {
+            code: self.top_level.code.or(nested.code),
+            message: self
+                .top_level
+                .message
+                .or(nested.message)
+                .unwrap_or_default(),
+            param: self.top_level.param.or(nested.param),
+        }
+    }
+}
+
 #[derive(Deserialize, Debug)]
 #[serde(tag = "type")]
 pub enum StreamEvent {
@@ -278,7 +315,7 @@ pub enum StreamEvent {
     #[serde(rename = "error")]
     GenericError {
         #[serde(flatten)]
-        error: ResponseError,
+        error: GenericStreamErrorPayload,
     },
     #[serde(other)]
     Unknown,