feat: add ping health check endpoint (#87)

README.md

@@ -331,6 +331,15 @@ stellar contract invoke --id $CONTRACT_ID --source-account <KEY> --network local
   -- get_version
 ```
 
+#### `ping`
+
+Health check endpoint. Returns the current ledger timestamp (u64).
+
+```bash
+stellar contract invoke --id $CONTRACT_ID --source-account <KEY> --network local \
+  -- ping
+```
+
 #### `upgrade`
 
 Upgrades the contract WASM in-place. Admin only. Existing storage and contract address are preserved.

contracts/payment-processing-contract/src/test.rs

@@ -131,6 +131,15 @@ fn test_get_version_after_set_admin() {
     assert_eq!(client.get_version(), 1);
 }
 
+#[test]
+fn test_ping() {
+    let (env, client) = setup();
+    env.ledger().with_mut(|li| {
+        li.timestamp = 12345;
+    });
+    assert_eq!(client.ping(), 12345);
+}
+
 // ── Merchant tests ────────────────────────────────────────────────────────────
 
 #[test]