LangSmith reads the Cost column from outputs.usage_metadata.total_cost,
but LangsmithLogger._prepare_log_data never wrote to that key. The
response_cost was already computed in StandardLoggingPayload but was
not forwarded to the outputs dict.
Inject usage_metadata with input_tokens, output_tokens, total_tokens,
and total_cost into the outputs dict so LangSmith can display cost.
Fixes#24001
Made-with: Cursor