test: add EQL regression test framework for backwards compatibility

Add regression tests to verify that data encrypted by previous proxy
versions can still be decrypted after upgrades. This is critical for
production deployments.

The test framework includes:
- Fixture generation (run on main branch with CS_GENERATE_EQL_FIXTURES=1)
- Regression tests for all data types (text, int2, int4, int8, float8, bool, jsonb)
- JSONB field access and array operation tests on legacy data

Tests gracefully skip when no fixtures are present, allowing CI to pass
while fixtures are generated separately.

Author: coderdan