Detect Banish via C_UnitAuras.GetUnitAuraBySpellID, not slot scan

The Banish immunity check hand-scanned 16 debuff slots plus 32 buff slots
(NPC overflow) for spellID 710/18647. ClassicAPI.GetUnitAuraBySpellID walks
the whole aura array in one call per rank, so it finds the debuff wherever it
lands -- including an NPC's overflow buff slots -- dropping the two loops and
the UnitIsPlayer gate. Wrapper added alongside UnitHasDispelType.

ClassicAPI.lua

@@ -82,6 +82,19 @@ function API.UnitHasDispelType(unit, dispelType, helpful)
     return scanDispel(indexFn, unit, dispelType, wantAny)
 end
 
+-- First matching aura on `unit` by spellID, or nil. With no filter it walks the
+-- whole aura array (helpful then harmful), so it finds a debuff even when it has
+-- overflowed into an NPC's buff slots -- no 16+32 slot scan, no UnitIsPlayer
+-- gate. filter ("HELPFUL"/"HARMFUL") restricts the search. Returns the modern
+-- AuraData (spellId, name, applications, duration, expirationTime, dispelName, ...).
+function API.GetUnitAuraBySpellID(unit, spellID, filter)
+    if not unit or not spellID then return nil end
+    if type(C_UnitAuras) ~= "table" or type(C_UnitAuras.GetUnitAuraBySpellID) ~= "function" then
+        return nil
+    end
+    return C_UnitAuras.GetUnitAuraBySpellID(unit, spellID, filter)
+end
+
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 -- GetUnitSpeed
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