Address review: complete the node-facing surface, wiring test, comment placement

- read_descriptor and notify_characteristic passthroughs join the frozen
  surface so the first migrated node can subscribe without extending the
  client; the comment now says which ops have callers today
- The choke-point test also validates through the public
  BLE_CLIENT_SCHEMA, so removing the cv.All wiring fails a test
- DOMAIN no longer sits between the BTstack comment and the constant it
  explains

tests/component_tests/ble_client/test_validation.py

@@ -106,3 +106,6 @@ def test_legacy_node_choke_point_rejects_other_platforms() -> None:
     CORE.data.setdefault(KEY_CORE, {})[KEY_TARGET_PLATFORM] = PLATFORM_RP2
     with pytest.raises(cv.Invalid, match="not been migrated"):
         ble_client._legacy_engine_only(ID("x"))
+    # Through the public schema too, so removing the cv.All wiring fails here.
+    with pytest.raises(cv.Invalid, match="not been migrated"):
+        ble_client.BLE_CLIENT_SCHEMA({})