Validate the wire format against node-smpp and add CI

test/reference-smpp.d.ts

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+/**
+ * Just enough of the reference implementation (farhadi/node-smpp) to cross-check the wire format.
+ * It ships no types of its own and is a dev dependency only.
+ */
+declare module 'smpp' {
+	type ReferencePdu = {
+		command: string;
+		command_status: number;
+		data_coding?: number;
+		destination_addr?: string;
+		esm_class?: number;
+		interface_version?: number;
+		message_id?: string;
+		message_state?: number;
+		password?: string;
+		receipted_message_id?: string;
+		sequence_number: number;
+		short_message?: { message: Buffer | string };
+		source_addr?: string;
+		system_id?: string;
+		toBuffer: () => Buffer;
+	};
+
+	type ReferenceSession = {
+		bind_transceiver: (options: Record<string, unknown>, cb: (pdu: ReferencePdu) => void) => void;
+		bind_transceiver_resp: (options: Record<string, unknown>) => void;
+		close: () => void;
+		on: (event: string, listener: (pdu: ReferencePdu) => void) => void;
+		submit_sm: (options: Record<string, unknown>, cb?: (pdu: ReferencePdu) => void) => void;
+		submit_sm_resp: (options: Record<string, unknown>) => void;
+	};
+
+	type ReferenceServer = {
+		address: () => { port: number } | null;
+		close: (cb?: () => void) => void;
+		listen: (port: number, cb?: () => void) => void;
+	};
+
+	const smpp: {
+		PDU: {
+			new (command: string, fields?: Record<string, unknown>): ReferencePdu;
+			fromBuffer: (buffer: Buffer) => ReferencePdu | false;
+		};
+		connect: (options: Record<string, unknown>, cb?: () => void) => ReferenceSession;
+		createServer: (
+			options: Record<string, unknown>,
+			handler: (session: ReferenceSession) => void,
+		) => ReferenceServer;
+	};
+
+	export default smpp;
+	export type { ReferencePdu, ReferenceServer, ReferenceSession };
+}