Closes the error-code gap between LangGraph and FastGPT backends: the
endpoint's 401/429 branches previously could never trigger under the
LangGraph backend because all provider failures collapsed into LLMAPIError
(500).
- agent/llm: LLMAuthError / LLMRateLimitError as LLMAPIError subclasses;
OpenAILLMService raises them from openai AuthenticationError (401, no
retry) and RateLimitError (429, after bounded retries exhausted)
- backends/langgraph: _complete_turn translates LLMAuthError ->
ChatBackendAuthenticationError and LLMRateLimitError ->
ChatBackendRateLimitError; stream reuses complete() so the mapping
applies to both paths; other errors (timeout etc.) propagate unchanged
- endpoint untouched: existing 401/429 response branches now fire for
both backends
Verified: 212 unit tests (7 new error-mapping tests)
Eric Wang
ca73b1e8c5