Adds a new getSubscriptions getter to ServerWebSocket that returns an array
of all topics the WebSocket is currently subscribed to.
Implementation details:
- Added getTopicsCount() and getTopics() methods to uWS WebSocket.h
- Created uws_ws_get_topics_as_js_array() in libuwsockets.cpp that constructs
a JSArray directly using JSC APIs, avoiding Zig FFI complexity
- Uses iterateTopics() internally to populate the array
- Exposed as a getter property in server.classes.ts
- Added comprehensive test coverage
The getter returns an empty array when there are no subscriptions.
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