docs(contract): clarify minimal OCS envelope + reuse _BROKER_READY

Address round-1 review nits:
- document why _ocs_capabilities omits the rest of the OCS envelope (Pact V4
  allows extra provider-side keys; pin only astrolabe's own block)
- use the module-level _BROKER_READY in the broker-source guard instead of
  re-checking the three env vars inline

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/contract/test_astrolabe_capabilities_consumer.py

@@ -36,7 +36,14 @@ pytestmark = pytest.mark.contract
 
 
 def _ocs_capabilities(enabled_doc_types: list[str]) -> dict:
-    """Minimal OCS envelope carrying just astrolabe's semantic_search block."""
+    """Minimal OCS envelope carrying just astrolabe's semantic_search block.
+
+    Intentionally omits the rest of a real OCS response (other apps'
+    capabilities, ``meta.statuscode``/``message``, etc.): Pact V4 allows extra
+    provider-side keys, so pinning only the block this consumer reads keeps the
+    contract focused on what astrolabe owns without coupling to Nextcloud-core
+    envelope fields.
+    """
     return {
         "ocs": {
             "meta": {"status": "ok"},