feat(e2e): add E2E test infrastructure

Add pytest fixtures and configuration for end-to-end testing with real
database operations. Includes module-scoped database fixtures, seeded
test data fixtures, and registry integration fixtures.

Implements Issue #43 Phase 1.

Author: Sakeeb91

tests/e2e/__init__.py

@@ -0,0 +1,16 @@
+"""
+End-to-end integration tests for Arctic Text2SQL Agent.
+
+These tests run with real model inference and database execution
+to verify the complete pipeline from natural language to SQL results.
+
+Usage:
+    # Run all E2E tests
+    pytest tests/e2e/ -v
+
+    # Run with specific markers
+    pytest tests/e2e/ -m "e2e and not slow" -v
+
+    # Skip E2E tests in regular test runs
+    pytest tests/ -m "not e2e" -v
+"""