Fix handling of server-initiated messages

[EliavLunar]

Fixes #155

Problem
The proxy would fail transferring server-initiated messages (like ping) to end-client as it tried to access properties on an undefined originalRequest, assuming all requests arrived from end-client originally (which is not necessarily true, e.g. the case of server-initiated pings).

Solution
Added an early return in interceptResponse (src/lib/utils.ts) when no matching request exists in pendingRequests, allowing these messages to pass through safely.

Testing
Added tests to verify the proxy correctly handles:

• Server-initiated requests (e.g., ping messages)
• Orphaned responses without matching client requests (not supposed to happen, but this is another way to look at this fix)