Pig saddle attachable change causes resource packs with old pigs to show 2 saddles

In 1.21.60.25 preview's "Drop 1 2025" experiment, the pig saddle has been changed to an attachable. The change to an attachable causes issues for existing resource packs and other bugs like MCPE-189750 and MCPE-189751.

Now, any resource pack which includes old pig render controllers, textures, and geometry will render two pig saddles instead of one. Historically, this method has allowed resource packs to lock in a mob appearance for future updates, but the rule is broken by this saddle change.

This also affects the Timeless Adventures world on the Marketplace when the experiment is enabled. Note that the broken model for the lower saddle is caused by MCPE-189751.

To Reproduce

1. Import and apply the sample resource pack in the latest preview: pig_duplicate_saddle.mcpack
2. Create a new world with the "Drop 1 2025" experiment enabled.
3. Summon a pig.
4. Equip a saddle on the pig.
5. Observe two saddles instead of one.

Expected Result

Resource packs with old pig textures, geometry, and render controllers configured will keep the old pig appearance.

Observed Result

Resource packs with old pig textures, geometry, and render controllers now render two saddles. The old "pig_saddle" texture is rendered at the same time as the redundant saddle attachable.

To Fix

The new saddle should be handled as a pig render controller, not an attachable, to avoid redundant saddle rendering. The sample fix pack attached to MCPE-189750 demonstrates how to do this.