fix(api): surface provider error body before attempting completion parse

When a local/proxy OpenAI-compatible backend returns an error object:
  {"error":{"message":"...","type":"...","code":...}}

claw was trying to deserialize it as a ChatCompletionResponse and
failing with the cryptic 'failed to parse OpenAI response: missing
field id', completely hiding the actual backend error message.

Fix: before full deserialization, check if the parsed JSON has an
'error' key and promote it directly to ApiError::Api so the user
sees the real error (e.g. 'The number of tokens to keep from the
initial prompt is greater than the context length').

Source: devilayu in #claw-code 2026-04-09 — local LM Studio context
limit error was invisible; user saw 'missing field id' instead.
159 CLI + 115 api tests pass. Fmt clean.

rust/crates/api/src/providers/openai_compat.rs

@@ -157,6 +157,35 @@ impl OpenAiCompatClient {
         let response = self.send_with_retry(&request).await?;
         let request_id = request_id_from_headers(response.headers());
         let body = response.text().await.map_err(ApiError::from)?;
+        // Some backends return {"error":{"message":"...","type":"...","code":...}}
+        // instead of a valid completion object. Check for this before attempting
+        // full deserialization so the user sees the actual error, not a cryptic
+        // "missing field 'id'" parse failure.
+        if let Ok(raw) = serde_json::from_str::<serde_json::Value>(&body) {
+            if let Some(err_obj) = raw.get("error") {
+                let msg = err_obj
+                    .get("message")
+                    .and_then(|m| m.as_str())
+                    .unwrap_or("provider returned an error")
+                    .to_string();
+                let code = err_obj
+                    .get("code")
+                    .and_then(|c| c.as_u64())
+                    .map(|c| c as u16);
+                return Err(ApiError::Api {
+                    status: reqwest::StatusCode::from_u16(code.unwrap_or(400))
+                        .unwrap_or(reqwest::StatusCode::BAD_REQUEST),
+                    error_type: err_obj
+                        .get("type")
+                        .and_then(|t| t.as_str())
+                        .map(str::to_owned),
+                    message: Some(msg),
+                    request_id,
+                    body,
+                    retryable: false,
+                });
+            }
+        }
         let payload = serde_json::from_str::<ChatCompletionResponse>(&body).map_err(|error| {
             ApiError::json_deserialize(self.config.provider_name, &request.model, &body, error)
         })?;