VertexAIAdcConfigLoader

Loads where gcloud keeps its Application Default Credentials file, at the configuration edge (#1939).

Reads the llm4s.vertexai.adc block (appData, cloudSdkConfig; llm4s-gemini's reference.conf binds them to ${?APPDATA} and ${?CLOUDSDK_CONFIG}) and the os.name and user.home system properties, and resolves them with org.llm4s.llmconnect.provider.GcloudAdcLocation.wellKnownPath. The result is injected into the client as VertexAIConfig.wellKnownCredentialPath; the authentication code reads no configuration itself.