Add ActiveBelongsToNotNull + nullable belongs_to detach

[tyt2y3]

Follow-up to #3110.

1. Rename (026dfe1e) — mechanical, word-boundary rename crate-wide:

   • HasOneModel -> ActiveHasOne
   • HasManyModel -> ActiveHasMany

   These are the active/write-side companions to HasOne/HasMany; the old *Model names read as loaded models. No behavior change.

2. ActiveBelongsToNotNull + nullable detach (af16e86f):

   • New type ActiveBelongsToNotNull<E> with variants NotSet / Set — no Delete. It is the active-side type for a non-nullable belongs_to, so trying to detach a relation whose self-FK cannot be nulled is now a compile error instead of a silent no-op / raw SQL constraint error at runtime.
   • The derive macro inspects each belongs_to's self-FK column nullability:
     • non-nullable FK -> field typed ActiveBelongsToNotNull (no delete_* / set_*_option builders generated)
     • nullable FK -> field typed ActiveHasOne, and Delete now detaches by setting the self-FK to NULL (previously a no-op)
   • from is snake_cased before lookup, so both a Column name (BakeryId) and a field name (user_id) resolve to the FK field. Composite / unresolvable from values fall back to ActiveHasOne with no detach (keeps them compiling).

Previously, calling delete/detach on a non-nullable belongs_to silently did nothing (or surfaced a raw DB constraint error). This makes the invariant type-level for belongs_to — the only relation whose FK nullability is visible to the derive macro — while keeping codegen concrete per field so downstream users don't need generic bounds.