Patch Changes

• #9763 8f0757f Thanks @AdamParker19! - fix(langchain): resolve className collision in MODEL_PROVIDER_CONFIG

  Refactored getChatModelByClassName to accept an optional modelProvider parameter for direct lookup, avoiding the className collision issue where multiple providers share the same className (e.g., google-vertexai and google-vertexai-web both use "ChatVertexAI"). When modelProvider is provided, the function uses direct config lookup instead of searching by className. Backward compatibility is maintained for existing callers that only pass className. This eliminates the duplicated import logic that was previously in _initChatModelHelper.