Policy: derive Proxy from WithConv, not Clause

scalafmt-core/shared/src/main/scala/org/scalafmt/internal/Policy.scala

@@ -356,30 +356,23 @@ object Policy {
 
   private class Proxy(policy: Policy, factory: Policy => Pf)(implicit
       fl: FileLine,
-  ) extends Clause {
+  ) extends WithConv {
     override val f: Pf = factory(policy)
     override def rank: Int = policy.rank
 
-    override def filter(pred: Clause => Boolean): Policy =
-      if (!pred(this)) NoPolicy else Proxy(policy.filter(pred))(factory)
-
-    override def exists(pred: Clause => Boolean): Boolean = policy.exists(pred)
-
-    override def switch(trigger: T, on: Boolean): Policy = {
-      val switched = policy.switch(trigger, on)
-      if (switched eq policy) this else Proxy(switched)(factory)
+    override protected def conv(pred: Policy => Policy): Policy = {
+      val filtered = pred(policy)
+      if (filtered eq policy) this else Proxy(filtered)(factory)
     }
 
-    override def unexpired(split: Split, nextft: FT): Policy =
-      Proxy(policy.unexpired(split, nextft))(factory)
+    override def exists(pred: Clause => Boolean): Boolean = policy.exists(pred)
 
     override def appliesUntil(nextft: FT)(pred: Clause => Boolean): Boolean =
       policy.appliesUntil(nextft)(pred)
 
     override def noDequeue: Boolean = policy.noDequeue
 
-    override val prefix: String = "*"
-    override def suffix: String = s"($policy)"
+    override def toString: String = s"*($policy)"
   }
 
   final class Map(