Bundle core inside the integration

HACS and the manual install copy only custom_components/nestquest, so
the integration shipped with a top-level 'core' package that was never
deployed and failed with ModuleNotFoundError: No module named 'core'.

Move core/ into custom_components/nestquest/core/, repoint the
re-export shims to relative '.core.X' imports, and update tests
(import paths, monkeypatch targets, caplog logger) to the new
'custom_components.nestquest.core.X' location. The integration is now
self-contained: nothing under custom_components/nestquest reaches
outside its own tree, the migrations logger becomes
custom_components.nestquest.core.migrations, and the SQL-leak guard
allowlists point at the new core/ paths.

uv run pytest -q: 1496 passed.

tests/test_migrations.py

@@ -707,7 +707,7 @@ def test_noop_run_logs_no_migration(caplog, tmp_path) -> None:
     try:
         _migrate(database)
         caplog.clear()
-        with caplog.at_level(logging.INFO, logger="core.migrations"):
+        with caplog.at_level(logging.INFO, logger="custom_components.nestquest.core.migrations"):
             _migrate(database)
         migrated = [
             r for r in caplog.records if "migrated" in r.getMessage().lower()
@@ -722,7 +722,7 @@ def test_noop_run_is_logged_at_debug_level(caplog, tmp_path) -> None:
     try:
         _migrate(database)
         caplog.clear()
-        with caplog.at_level(logging.DEBUG, logger="core.migrations"):
+        with caplog.at_level(logging.DEBUG, logger="custom_components.nestquest.core.migrations"):
             _migrate(database)
         assert any(
             f"already at version {len(MIGRATIONS)}" in r.getMessage()