Comprehensive test coverage for relations (select_related / load_related)

[1st]

Why

select_related and load_related are core ORM features but currently have 2 tests each, covering only the forward to-one happy path. This is the highest-regression-risk area before 1.0.

Cases to cover (both Postgres + MySQL)

• select_related (JOIN): forward to-one; multiple forward relations in one call; NULL FK → relation is None; missing/None related row; composition with filter/order_by/limit.
• load_related (batched): forward, reverse (list[...]), nested (a__b); empty result set; object with no related rows → empty list / None; multiple roots sharing a related object (dedupe); N+1 avoidance (one query per relation level).
• Cross-cutting: explicit ValueError when only()/defer() combined with select_related() (guard added in Load only given subset of fields from DB #50); relation fields excluded from to_dict() by default.

Done when

Each case above is a test, green on both backends; any bug surfaced is filed as its own issue and fixed.