Improve upstream error reporting (#34668)

Now we handle more upstream error cases using the same auto-retry logic.

Release Notes:

- N/A

crates/language_model/src/language_model.rs

@@ -116,6 +116,12 @@ pub enum LanguageModelCompletionError {
         provider: LanguageModelProviderName,
         message: String,
     },
+    #[error("{message}")]
+    UpstreamProviderError {
+        message: String,
+        status: StatusCode,
+        retry_after: Option<Duration>,
+    },
     #[error("HTTP response error from {provider}'s API: status {status_code} - {message:?}")]
     HttpResponseError {
         provider: LanguageModelProviderName,