polish: IsValidEntity guards on entity-scoped overloads + SetOneShot(bool)

Pre-merge defensive hygiene (from MERGE_READINESS.md): every entity-scoped Hook/
SetEntity/Unhook overload now guards 'entity is not { IsValidEntity: true }' before
reading entity.Index (a stale ModSharp entity wrapper is a dangling native pointer);
BeginSendFake's null check upgraded to the same. Added a SetOneShot(bool) overload for
parity with the rest of the typed spoof API. Builds clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>