test: add configuration loading tests

- Add test_config.py with tests for load_settings function
- Add test_config.yaml fixture with test database configuration
- Tests cover: successful loading, file not found error, database URL generation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/fixtures/test_config.yaml

@@ -0,0 +1,10 @@
+# 测试配置
+database:
+  sql_server:
+    host: localhost
+    port: 1433
+    database: TestDB
+    username: test_user
+    password: test_pass
+    driver: "{ODBC Driver 18 for SQL Server}"
+    trust_server_certificate: yes