Add SelectFourMany with consolidate() for star topology (#3054)

[tyt2y3]

Staging PR for #3054 (preserves @TonySchneider's authorship + merged status per our branch policy for external contributions).

@TonySchneider's feature (merged into this branch)

Adds SelectFourMany<E, F, G, H, TOP>, SelectFour::consolidate(), and SelectFourMany<TopologyStar>::all() — mirroring SelectThreeMany — so a parent fetched with three has-many children in one query consolidates into Vec<(E::Model, Vec<F::Model>, Vec<G::Model>, Vec<H::Model>)> instead of flat Option tuples. Reuses retain_unique_models / ModelKey; star topology only (chain/tee can follow). Purely additive.

Staging work (this branch)

• Reviewed against the SelectThreeMany/tee patterns: consolidation logic is faithful (correct first-occurrence ordering, childless-parent handling, per-child dedup). SelectFour is only ever TopologyStar (sole constructor is SelectThree::find_also), so the Star-only all() covers every reachable case; SelectFourMany is exported via pub use select::*.
• Added a runtime test (tests/select_four_many_tests.rs): builds a 4-way star (center -> a, b, c), runs find_also_related ×2 → find_also → consolidate().all(), and checks the a×b×c cross product consolidates back to one tuple per parent. Uses order_by_asc on each child for deterministic order (matches the existing SelectThree consolidate test). Verified on SQLite, PostgreSQL, and rusqlite (sync).
• Regenerated sea-orm-sync (the PR only touched src/).