fix: detect HTML responses in streaming path

When a provider returns HTML (e.g., error page, wrong endpoint) instead
of JSON in an SSE stream, provide a clear error message instead of
hanging or failing with a cryptic parse error.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -1588,6 +1588,7 @@ fn parse_sse_frame(
             body: payload.chars().take(200).collect(),
             retryable: false,
             suggested_action: Some("verify the API endpoint URL is correct".to_string()),
+            retry_after: None,
         });
     }
     serde_json::from_str::<ChatCompletionChunk>(&payload)