Gives Geras Dionae melee armour (#20750)

As title, requested by Diona lore with the intention of making Geras
more viable fighting simplemobs - particularly if you want to play one
as a miner.

This is the same rank of armour as a Vaurca Bulwark, though missing
their substantive brute modifier. This has been implemented as melee
armour instead of a brute modifier so they aren't any less susceptible
to projectiles.

Doesn't apply to Coeus Dionae.

code/unit_tests/mob_tests.dm

@@ -420,6 +420,7 @@
 /datum/unit_test/mob_damage/diona/brute
 	name = "MOB: Diona Brute Damage Check"
 	damagetype = DAMAGE_BRUTE
+	expected_vulnerability = ARMORED
 
 /datum/unit_test/mob_damage/diona/fire
 	name = "MOB: Diona Fire Damage Check"