Make DefaultWeaponAssetPath safer for NPC default weapons

[ifBars]

Problem

CombatBehaviour.DefaultWeaponAssetPath accepts raw string resource paths for NPC default weapons. Developers can accidentally guess paths such as Weapon/Name instead of using real base-game equippable resource names.

The safer APIs already exist in S1API/Entities/Behaviour/CombatBehaviour.cs:

• SetDefaultWeapon(Equippable)
• SetDefaultWeapon(EquippableBuilder)

Desired behavior

Make direct DefaultWeaponAssetPath usage safer by doing one of the following:

1. Validate that non-empty string values resolve to a real loadable weapon/equippable resource in the base game.
2. Warn developers when they set DefaultWeaponAssetPath directly, pointing them to the type-safe SetDefaultWeapon(Equippable) and SetDefaultWeapon(EquippableBuilder) overloads for reliability.

Acceptance criteria

• Invalid guessed weapon paths are surfaced clearly instead of failing silently or leaving developers guessing.
• The warning/error message points developers toward the type-safe default-weapon APIs.
• Existing valid weapon-path behavior remains compatible unless intentionally deprecated.