test(subscription): bring src/api/subscription to 100% offline line coverage

tests/api/subscription/_mockTransport.ts

@@ -0,0 +1,45 @@
+/**
+ * Mock transport for offline Subscription API tests.
+ *
+ * Records every `subscribe(...)` call — channel, validated payload, wrapped listener and
+ * options — and resolves with a stub {@linkcode ISubscription}, so tests can assert exactly
+ * what the API layer sent and replay synthetic events through the captured listener.
+ *
+ * @module
+ */
+
+import type { ISubscription, ISubscriptionTransport, TransportError } from "@bloxwap/hyperliquid";
+
+/** Subscription options as seen by the transport. */
+export interface MockSubscribeOptions {
+  signal?: AbortSignal;
+  onError?: (error: TransportError) => void;
+}
+
+/** One recorded subscription, after request-schema validation (so `type` is always set). */
+export interface MockSubscribeCall {
+  channel: string;
+  payload: unknown;
+  listener: (data: CustomEvent<unknown>) => void;
+  options?: MockSubscribeOptions;
+}
+
+/** An {@linkcode ISubscriptionTransport} that records subscriptions instead of opening a socket. */
+export class MockSubscriptionTransport implements ISubscriptionTransport {
+  readonly calls: MockSubscribeCall[] = [];
+
+  subscribe<T>(
+    channel: string,
+    payload: unknown,
+    listener: (data: CustomEvent<T>) => void,
+    options?: MockSubscribeOptions,
+  ): Promise<ISubscription> {
+    this.calls.push({ channel, payload, listener: listener as (data: CustomEvent<unknown>) => void, options });
+    return Promise.resolve({ unsubscribe: () => Promise.resolve() });
+  }
+
+  /** Replays a server event to the listener captured by the `index`-th subscribe call (default: last). */
+  emit(detail: unknown, index: number = this.calls.length - 1): void {
+    this.calls[index].listener({ detail } as CustomEvent<unknown>);
+  }
+}