[1.21.1 NeoForge] Fix custom horse armor textures

[Campione01]

Problem

Minecraft 1.21.1 AnimalArmorItem.BodyType.EQUESTRIAN derives the body texture from the armor material registry key as:

textures/entity/horse/armor/horse_armor_<material-key>.png

The four Middle-earth horse armor textures still used the Minecraft 1.21.8 equipment path, so horses rendered them as missing textures on the NeoForge 1.21.1 branch.

Fix

• Move the Gondorian, Rohirric, Dalish, and Lorien horse armor textures to their exact 1.21.1 runtime paths.
• Keep all Warg and Broadhoof beast_armor textures unchanged because their custom renderers load those paths directly.
• Add source-resource and packaged-JAR regression tests.
• Extend CI to require the four 1.21.1 entries and reject the incompatible 1.21.8 entries.

Validation

• ./gradlew clean build --console=plain passes.
• 118 tests in 26 suites pass with no failures, errors, or skips.
• The player JAR contains all four expected horse armor textures and none of the four incompatible paths.
• The 13 custom Warg/Broadhoof armor textures remain packaged.
• Exact-head CI passed at ba36abd51.

No existing upstream or fork issue matched this NeoForge 1.21.1 defect.

[Jukoz]

Good horse armor fixes! Thank you