fix: handle orphan subscription messages during WebSocket reconnection

[demora33]

Fixes #547

Problem

During WebSocket reconnection, handleMessage() can crash due to timing-related race conditions when in-flight messages arrive before subscriptions are fully restored.

Crash 1: Malformed messages (line 500)

TypeError: Cannot read properties of undefined (reading 'subscription')
at AlchemyWebSocketProvider.handleMessage (alchemy-websocket-provider.ts:500:39)

Some messages pass isSubscriptionEvent() but do not contain a params object.
Since isSubscriptionEvent() only checks that the message is not a response (id is undefined), accessing message.params.subscription can throw.

Crash 2: Orphan subscription messages (line ~505)

TypeError: Cannot read properties of undefined (reading 'method')

This occurs when a virtualId still exists in virtualIdsByPhysicalId but the corresponding entry has already been removed from virtualSubscriptionsById

The existing non-null assertion (!) causes a crash when accessing subscription.method.

Both issues are race conditions that happen during the reconnection window while SturdyWebSocket is re-establishing the connection and re-subscribing.

Root Cause Analysis Timeline:

1. WebSocket is connected with active subscriptions
2. Network interruption causes disconnection
3. Alchemy server still has in-flight messages
4. SturdyWebSocket reconnects automatically
5. handleReopen() clears and rebuilds subscription mappings
6. An in-flight message arrives during this window
7. handleMessage() processes the message while internal state is inconsistent
8. Accessing undefined properties throws a TypeError