Add on.error handler to AgentClient lifecycle events

[Copilot]

The WsClient emits error events, but AgentClient.on had no typed handler for them—users could only access errors via the wildcard on.any handler.

Changes

• Added on.error lifecycle handler to AgentClient.on object, matching the pattern of on.connected and on.disconnected

Usage

const client = new AgentClient({ apiKey: 'your-key' });
client.connect();

// Now available alongside other lifecycle handlers
client.on.error(() => {
  console.log('WebSocket error occurred');
});

client.on.connected(() => { /* ... */ });
client.on.disconnected(() => { /* ... */ });
client.on.reconnecting((attempt) => { /* ... */ });

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