refactor: change SubscriptionInfo.contract_key to ContractInstanceId

Subscriptions are identified by instance ID, not full ContractKey.
This aligns with the GET/SUBSCRIBE refactor that uses ContractInstanceId.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: sanity

rust/src/client_api/client_events.rs

@@ -798,7 +798,7 @@ pub struct SystemMetrics {
 
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct SubscriptionInfo {
-    pub contract_key: ContractKey,
+    pub contract_key: ContractInstanceId,
     pub client_id: usize,
 }