get authMethod() {
    const method = process.env.AWS_BEDROCK_LLM_CONNECTION_METHOD || "iam";
    return SUPPORTED_CONNECTION_METHODS.includes(method) ? method : "iam";
  }
Gets the configured AWS authentication method ('iam' or 'sessionToken').
Defaults to 'iam' if the environment variable is invalid.
@returns {"iam" | "iam_role" | "sessionToken"} The authentication method.