test: lock in instance-attribute reads in api_bootstrapper_config

Port the regression test from litestar-sqlalchemy-template#32. The
production bug fixed there (api_bootstrapper_config reading the module
-global `settings` singleton instead of `self`) is already absent here,
but there was no test guarding against it being reintroduced.

The test constructs a non-singleton Settings instance and asserts its
api_bootstrapper_config reflects that instance, not the global.

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

Author: lesnik512

tests/test_settings.py

@@ -0,0 +1,10 @@
+from app.settings import Settings
+
+
+def test_api_bootstrapper_config_reads_instance_not_global() -> None:
+    custom = Settings(service_name="custom-service", service_version="9.9.9")
+
+    config = custom.api_bootstrapper_config
+
+    assert config.service_name == "custom-service"
+    assert config.service_version == "9.9.9"