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/coordinator-api/tests/test_marketplace.py

@@ -0,0 +1,113 @@
+import pytest
+from fastapi.testclient import TestClient
+from sqlmodel import Session, delete
+
+from app.config import settings
+from app.domain import MarketplaceOffer, OfferStatus, MarketplaceBid
+from app.main import create_app
+from app.services.marketplace import MarketplaceService
+from app.storage.db import init_db, session_scope
+
+
+@pytest.fixture(scope="module", autouse=True)
+def _init_db(tmp_path_factory):
+    db_file = tmp_path_factory.mktemp("data") / "marketplace.db"
+    settings.database_url = f"sqlite:///{db_file}"
+    init_db()
+    yield
+
+
+@pytest.fixture()
+def session():
+    with session_scope() as sess:
+        sess.exec(delete(MarketplaceBid))
+        sess.exec(delete(MarketplaceOffer))
+        sess.commit()
+        yield sess
+
+
+@pytest.fixture()
+def client():
+    app = create_app()
+    return TestClient(app)
+
+
+def test_list_offers_filters_by_status(client: TestClient, session: Session):
+    open_offer = MarketplaceOffer(provider="Alpha", capacity=250, price=12.5, sla="99.9%", status=OfferStatus.open)
+    reserved_offer = MarketplaceOffer(provider="Beta", capacity=100, price=15.0, sla="99.5%", status=OfferStatus.reserved)
+    session.add(open_offer)
+    session.add(reserved_offer)
+    session.commit()
+
+    # All offers
+    resp = client.get("/v1/marketplace/offers")
+    assert resp.status_code == 200
+    payload = resp.json()
+    assert len(payload) == 2
+
+    # Filter by status
+    resp_open = client.get("/v1/marketplace/offers", params={"status": "open"})
+    assert resp_open.status_code == 200
+    open_payload = resp_open.json()
+    assert len(open_payload) == 1
+    assert open_payload[0]["provider"] == "Alpha"
+
+    # Invalid status yields 400
+    resp_invalid = client.get("/v1/marketplace/offers", params={"status": "invalid"})
+    assert resp_invalid.status_code == 400
+
+
+def test_marketplace_stats(client: TestClient, session: Session):
+    session.add_all(
+        [
+            MarketplaceOffer(provider="Alpha", capacity=200, price=10.0, sla="99.9%", status=OfferStatus.open),
+            MarketplaceOffer(provider="Beta", capacity=150, price=20.0, sla="99.5%", status=OfferStatus.open),
+            MarketplaceOffer(provider="Gamma", capacity=90, price=12.0, sla="99.0%", status=OfferStatus.reserved),
+        ]
+    )
+    session.commit()
+
+    resp = client.get("/v1/marketplace/stats")
+    assert resp.status_code == 200
+    stats = resp.json()
+    assert stats["totalOffers"] == 3
+    assert stats["openCapacity"] == 350
+    assert pytest.approx(stats["averagePrice"], rel=1e-3) == 15.0
+    assert stats["activeBids"] == 0
+
+
+def test_submit_bid_creates_record(client: TestClient, session: Session):
+    payload = {
+        "provider": "Alpha",
+        "capacity": 120,
+        "price": 13.5,
+        "notes": "Need overnight capacity",
+    }
+    resp = client.post("/v1/marketplace/bids", json=payload)
+    assert resp.status_code == 202
+    response_payload = resp.json()
+    assert "id" in response_payload
+
+    bid = session.get(MarketplaceBid, response_payload["id"])
+    assert bid is not None
+    assert bid.provider == payload["provider"]
+    assert bid.capacity == payload["capacity"]
+    assert bid.price == payload["price"]
+    assert bid.notes == payload["notes"]
+
+
+def test_marketplace_service_list_offers_handles_limit_offset(session: Session):
+    session.add_all(
+        [
+            MarketplaceOffer(provider="A", capacity=50, price=9.0, sla="99.0%", status=OfferStatus.open),
+            MarketplaceOffer(provider="B", capacity=70, price=11.0, sla="99.0%", status=OfferStatus.open),
+            MarketplaceOffer(provider="C", capacity=90, price=13.0, sla="99.0%", status=OfferStatus.open),
+        ]
+    )
+    session.commit()
+
+    service = MarketplaceService(session)
+    limited = service.list_offers(limit=2, offset=1)
+    assert len(limited) == 2
+    # Offers ordered by created_at descending → last inserted first
+    assert {offer.provider for offer in limited} == {"B", "A"}