fix(openai): improve Responses API stream robustness and diagnostics

- Implement final buffer flush in streaming path to prevent data loss
- Increase probe response body logging to 500 characters
- Ensure internal metadata is stripped even on final flush
- Fix potential hang when stream ends without explicit [DONE] event

src/providers/openai.rs

@@ -907,8 +907,9 @@ impl super::Provider for OpenAIProvider {
                                 let status = r.status();
                                 let body = r.text().await.unwrap_or_default();
                                 if status.is_success() {
-                                    tracing::warn!("Responses stream ended prematurely but probe returned 200 OK. Body: {}", body);
-                                    Err(AppError::ProviderError(format!("Responses stream ended (server sent 200 OK with body: {})", body)))?;
+                                    let preview = if body.len() > 500 { format!("{}...", &body[..500]) } else { body.clone() };
+                                    tracing::warn!("Responses stream ended prematurely but probe returned 200 OK. Body: {}", preview);
+                                    Err(AppError::ProviderError(format!("Responses stream ended (server sent 200 OK with body: {})", preview)))?;
                                 } else {
                                     tracing::error!("OpenAI Responses Stream Error Probe ({}): {}", status, body);
                                     Err(AppError::ProviderError(format!("OpenAI Responses API error ({}): {}", status, body)))?;
@@ -922,6 +923,21 @@ impl super::Provider for OpenAIProvider {
                     }
                 }
             }
+
+            // Final flush of content_buffer if not empty
+            if !content_buffer.is_empty() {
+                let stripped = Self::strip_internal_metadata(&content_buffer);
+                if !stripped.is_empty() {
+                    yield ProviderStreamChunk {
+                        content: stripped,
+                        reasoning_content: None,
+                        finish_reason: None,
+                        tool_calls: None,
+                        model: model.clone(),
+                        usage: None,
+                    };
+                }
+            }
         };
 
         Ok(Box::pin(stream))