fix: consolidate chat-completions compatibility fixes

- apply default mapped model only when scheduling fallback is actually used

- preserve reasoning in OpenAI-compatible output via reasoning_content and avoid invalid input function_call ids

backend/internal/pkg/apicompat/anthropic_to_responses.go

@@ -277,7 +277,6 @@ func anthropicAssistantToResponses(raw json.RawMessage) ([]ResponsesInputItem, e
 			CallID:    fcID,
 			Name:      b.Name,
 			Arguments: args,
-			ID:        fcID,
 		})
 	}