feat(endtoend): add engine-specific e2e test packages

[kyleconroy]

Add a new end-to-end test structure with separate packages for each SQL engine (PostgreSQL, MySQL, SQLite). Each engine has:

• Its own test runner (endtoend_test.go)
• Coverage verification test (coverage_test.go)
• Engine-specific schema with correct SQL syntax
• Sample test cases demonstrating the pattern

The testcases package defines a registry of ~120 test cases that each engine should implement, with capabilities tracking to handle engine-specific features (RETURNING, FULL OUTER JOIN, enums, etc.).

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]