Hilarious things you've found in RAW?

[Promethean]

In both cases they "are treated as," not "are," constructs. Intelligent weapons don't have HD and neither do flying weapons. Item/creature is exclusive with the other mechanically.

There's literally an entire creature species called a "Symbiont" that disagrees with this. They're able to provide enhancement bonuses and spell charges just as any magic item as well. Example:

Damage Reduction (Ex): A living breastplate has
damage reduction 10/byeshk and imparts this benefi t
upon its host.
Enhance Constitution (Su): A living breastplate
provides its owner with a +4 enhancement bonus to Constitution for as long as it is worn.
Light Fortifi cation (Ex): A living breastplate has
light fortifi cation (25% chance to negate any critical hit
or sneak attack) and imparts this ability to its host.
Stabilize Host (Su): A living breastplate can stabilize
a host that has dropped to negative hit points. This is a free
action that deals 1 point of Strength damage to the host.
The armor can do this without prompting from the host,
and will generally do so whenever the situation arises.

The entire "Magic Fang" line of spells also disagree with this, as the entire point of that is allowing creatures to count as both the user and the item for the purposes of enhancement bonuses and other effects. They can be made permanent as well.

Is Dark Sun 1st party? If so, They have an entire line of living artifacts, many with their own HD, in the life shapers handbook and strewn throughout other supplemental material.

There is no mechanic or rule that prevents a living creature from functioning as a magic item. In fact, there's much evidence to the opposite.