export declare abstract class BaseConnector {
    protected readonly fetchImpl: typeof fetch;
    protected constructor(fetchImpl?: typeof fetch);
    protected sendPostJson(url: string, body: unknown, options?: {
        headers?: Record<string, string>;
        query?: Record<string, string>;
    }): Promise<Response>;
    /**
     * Form-encoded POST with multi-value entry support — array values produce
     * repeated `key=` fields (e.g. Twilio expects multiple `MediaUrl=` fields
     * for MMS, not a comma-joined value). `null`/`undefined` entries are
     * skipped; everything else is `String()`-coerced.
     */
    protected sendPostForm(url: string, form: Record<string, unknown>, options?: {
        headers?: Record<string, string>;
        query?: Record<string, string>;
    }): Promise<Response>;
    private invokeFetch;
}