Based on the repository's commit message style and the changes in the diff, here's an appropriate commit message:

```
feat: add websocket tests, PoA metrics, marketplace endpoints, and enhanced observability

- Add comprehensive websocket tests for blocks and transactions streams including multi-subscriber and high-volume scenarios
- Extend PoA consensus with per-proposer block metrics and rotation tracking
- Add latest block interval gauge and RPC error spike alerting
- Enhance mock coordinator

apps/blockchain-node/tests/test_websocket.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+from contextlib import ExitStack
 
 from fastapi.testclient import TestClient
 
@@ -10,8 +11,6 @@ from aitbc_chain.gossip import gossip_broker
 
 def _publish(topic: str, message: dict) -> None:
     asyncio.run(gossip_broker.publish(topic, message))
-
-
 def test_blocks_websocket_stream() -> None:
     client = TestClient(create_app())
 
@@ -28,19 +27,100 @@ def test_blocks_websocket_stream() -> None:
         assert message == payload
 
 
-def test_transactions_websocket_stream() -> None:
+def test_blocks_websocket_multiple_subscribers_receive_all_payloads() -> None:
+    with TestClient(create_app()) as client, ExitStack() as stack:
+        sockets = [
+            stack.enter_context(client.websocket_connect("/rpc/ws/blocks"))
+            for _ in range(3)
+        ]
+
+        payloads = [
+            {
+                "height": height,
+                "hash": "0x" + f"{height:064x}",
+                "parent_hash": (
+                    "0x" + f"{height - 1:064x}" if height > 0 else "0x" + "0" * 64
+                ),
+                "timestamp": f"2025-01-01T00:00:{height:02d}Z",
+                "tx_count": height % 3,
+            }
+            for height in range(5)
+        ]
+
+        for payload in payloads:
+            _publish("blocks", payload)
+
+        for socket in sockets:
+            received = [socket.receive_json() for _ in payloads]
+            assert received == payloads
+
+        # Publish another payload to ensure subscribers continue receiving in order.
+        final_payload = {
+            "height": 99,
+            "hash": "0x" + "f" * 64,
+            "parent_hash": "0x" + "e" * 64,
+            "timestamp": "2025-01-01T00:01:39Z",
+            "tx_count": 5,
+        }
+        _publish("blocks", final_payload)
+
+        for socket in sockets:
+            assert socket.receive_json() == final_payload
+
+
+def test_blocks_websocket_high_volume_load() -> None:
+    message_count = 40
+    subscriber_count = 4
+
+    with TestClient(create_app()) as client, ExitStack() as stack:
+        sockets = [
+            stack.enter_context(client.websocket_connect("/rpc/ws/blocks"))
+            for _ in range(subscriber_count)
+        ]
+
+        payloads = []
+        for height in range(message_count):
+            payload = {
+                "height": height,
+                "hash": "0x" + f"{height + 100:064x}",
+                "parent_hash": "0x" + f"{height + 99:064x}" if height > 0 else "0x" + "0" * 64,
+                "timestamp": f"2025-01-01T00:{height // 60:02d}:{height % 60:02d}Z",
+                "tx_count": height % 7,
+            }
+            payloads.append(payload)
+            _publish("blocks", payload)
+
+        for socket in sockets:
+            received = [socket.receive_json() for _ in payloads]
+            assert received == payloads
+
+
+def test_transactions_websocket_cleans_up_on_disconnect() -> None:
     client = TestClient(create_app())
 
     with client.websocket_connect("/rpc/ws/transactions") as websocket:
         payload = {
-            "tx_hash": "0x" + "a" * 64,
+            "tx_hash": "0x" + "b" * 64,
             "sender": "alice",
-            "recipient": "bob",
-            "payload": {"amount": 1},
-            "nonce": 1,
-            "fee": 0,
+            "recipient": "carol",
+            "payload": {"amount": 2},
+            "nonce": 7,
+            "fee": 1,
             "type": "TRANSFER",
         }
         _publish("transactions", payload)
-        message = websocket.receive_json()
-        assert message == payload
+        assert websocket.receive_json() == payload
+
+    # After closing the websocket, publishing again should not raise and should not hang.
+    _publish(
+        "transactions",
+        {
+            "tx_hash": "0x" + "c" * 64,
+            "sender": "alice",
+            "recipient": "dave",
+            "payload": {"amount": 3},
+            "nonce": 8,
+            "fee": 1,
+            "type": "TRANSFER",
+        },
+    )