fix(animation): type key explicitly so Godot 4.6 parser accepts it

`var key := v.to_lower()` failed static type inference because `v` is
typed `Variant` and GDScript 4.6 can't resolve the call's return type.
Cast to String explicitly. Fixes the compile failure that blocked plugin
load on Linux/macOS/Windows Godot test jobs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dsarno

plugin/addons/godot_ai/handlers/animation_handler.gd

@@ -757,7 +757,7 @@ static func _parse_transition(v: Variant) -> float:
 	if v is float or v is int:
 		return float(v)
 	if v is String:
-		var key := v.to_lower()
+		var key: String = (v as String).to_lower()
 		if _NAMED_TRANSITIONS.has(key):
 			return float(_NAMED_TRANSITIONS[key])
 	return 1.0