Freeform stopReasons in sampling responses are unsupported

[LucaButBoring]

Bug description
The spec defines some well-known stopReason values, but also allows arbitrary values to be passed as well (spec). When parsing these, the Java SDK throws an exception:

com.fasterxml.jackson.databind.exc.InvalidFormatException: Cannot deserialize value of type `io.modelcontextprotocol.spec.McpSchema$CreateMessageResult$StopReason` from String "arbitrary value": not one of the values accepted for Enum class: [endTurn, stopSequence, maxTokens]

Environment
JDK21

Steps to reproduce
See PR #327

Expected behavior
SDK does not throw

Minimal Complete Reproducible example
See PR #327