fix(qdrant): verify collection existence in the readiness check

Why:
- The chunks collection is now created by an explicit deployment step
  (qdrant_bootstrap), not at startup, which means a process can boot against
  a healthy Qdrant that has no collection at all. /readyz's previous check
  only called get_collections(), so it reported ready in that state — the
  misconfiguration stayed invisible until the first upload failed with a 502
  after already paying for the MinIO write and embedding round trips.

Changes:
- ping_qdrant() now checks collection_exists(collection) instead of just
  reachability.

tests/integration/qdrant/test_readiness.py

@@ -0,0 +1,45 @@
+"""`/readyz`'s Qdrant probe against a real Qdrant.
+
+The case worth an integration test is the one a fake cannot produce
+convincingly: Qdrant is up and answering, but the collection the deployment
+step was supposed to create is not there.
+"""
+
+import uuid
+
+import pytest
+from qdrant_client import AsyncQdrantClient
+
+from src.config import QdrantSettings
+from src.infrastructure.qdrant.client import ping
+from src.infrastructure.qdrant.collection import ensure_chunks_collection
+
+pytestmark = [
+    pytest.mark.integration,
+    pytest.mark.qdrant,
+    pytest.mark.asyncio(loop_scope="session"),
+]
+
+
+async def test_ping_is_not_ready_when_the_collection_was_never_bootstrapped(
+    qdrant_client: AsyncQdrantClient, qdrant_settings: QdrantSettings
+) -> None:
+    """A healthy Qdrant with no collection is *not* ready: uploads would 502."""
+    assert await ping(qdrant_client, 5.0, collection=qdrant_settings.collection) is False
+
+
+async def test_ping_is_ready_after_bootstrap(
+    qdrant_client: AsyncQdrantClient, qdrant_settings: QdrantSettings
+) -> None:
+    await ensure_chunks_collection(qdrant_client, collection=qdrant_settings.collection)
+
+    assert await ping(qdrant_client, 5.0, collection=qdrant_settings.collection) is True
+
+
+async def test_ping_is_not_ready_for_a_different_collection_name(
+    qdrant_client: AsyncQdrantClient, qdrant_settings: QdrantSettings
+) -> None:
+    """A misconfigured QDRANT_COLLECTION is as unready as a missing one."""
+    await ensure_chunks_collection(qdrant_client, collection=qdrant_settings.collection)
+
+    assert await ping(qdrant_client, 5.0, collection=f"absent_{uuid.uuid4().hex}") is False