fix: add CI comment for profile flag and normalize conn string defaults

- Add comment explaining why --profile full is needed (ClickHouse is
  profile-gated; only explicitly named services start)
- Add `or None` to Databricks and MsSQL conn strings to handle empty
  env vars consistently with all other optional databases

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dtsong

.github/workflows/ci.yml

@@ -42,6 +42,7 @@ jobs:
         run: uv tool run ty check --python-version 3.10
 
       - name: Build the stack
+        # --profile full unlocks profile-gated clickhouse; only named services start
         run: docker compose --profile full up -d --wait mysql postgres clickhouse
 
       - name: Run tests

tests/common.py

@@ -27,12 +27,12 @@
 TEST_BIGQUERY_CONN_STRING: str = os.environ.get("DATADIFF_BIGQUERY_URI") or None
 TEST_REDSHIFT_CONN_STRING: str = os.environ.get("DATADIFF_REDSHIFT_URI") or None
 TEST_ORACLE_CONN_STRING: str = None
-TEST_DATABRICKS_CONN_STRING: str = os.environ.get("DATADIFF_DATABRICKS_URI")
+TEST_DATABRICKS_CONN_STRING: str = os.environ.get("DATADIFF_DATABRICKS_URI") or None
 TEST_TRINO_CONN_STRING: str = os.environ.get("DATADIFF_TRINO_URI") or None
 TEST_CLICKHOUSE_CONN_STRING: str = os.environ.get("DATADIFF_CLICKHOUSE_URI") or None
 TEST_VERTICA_CONN_STRING: str = os.environ.get("DATADIFF_VERTICA_URI") or None
 TEST_DUCKDB_CONN_STRING: str = "duckdb://main:@:memory:"
-TEST_MSSQL_CONN_STRING: str = os.environ.get("DATADIFF_MSSQL_URI")
+TEST_MSSQL_CONN_STRING: str = os.environ.get("DATADIFF_MSSQL_URI") or None
 
 
 DEFAULT_N_SAMPLES = 50