fix multiPart upload codec

[uryyyyyyy]

when I use multipart upload, it shows error like this.

[info] - Endpoint[echo raw parts](securityin: -, in: POST /api /echo /multipart {body as multipart/form-data}, errout: -, out: {body as text/plain (UTF-8)}) *** FAILED *** (2 milliseconds)
[info]   java.lang.ClassCastException: class sttp.model.Part cannot be cast to class scala.collection.immutable.List (sttp.model.Part is in unnamed module of loader sbt.internal.LayeredClassLoader @9cb3542; scala.collection.immutable.List is in unnamed module of loader sbt.internal.ScalaLibraryClassLoader @13aa12e4)
[info]   at sttp.tapir.Mapping$$anon$4.encode(Mapping.scala:60)
[info]   at sttp.tapir.Codec$$anon$1.encode(Codec.scala:75)
[info]   at sttp.tapir.Codec$$anon$2.encode(Codec.scala:93)
[info]   at sttp.tapir.Codec$$anon$5.$anonfun$encode$2(Codec.scala:212)
[info]   at scala.collection.immutable.List.flatMap(List.scala:293)
[info]   at sttp.tapir.Codec$$anon$5.$anonfun$encode$1(Codec.scala:211)
[info]   at scala.collection.immutable.List.flatMap(List.scala:293)
[info]   at sttp.tapir.Codec$$anon$5.encode(Codec.scala:210)
[info]   at sttp.tapir.Codec$$anon$5.encode(Codec.scala:205)
[info]   at sttp.tapir.Codec$$anon$1.encode(Codec.scala:75)
[info]   ...