Add session join schema and enhance error serialization

Introduces a Zod schema for the Minecraft session join endpoint, validating accessToken, selectedProfile, and serverId. Updates ValidationError to support serialization via YggdrasilError and SessionError for improved error formatting.

errors/ValidationError.js

@@ -1,5 +1,6 @@
-const path = require("node:path")
 const DefaultError = require("./DefaultError")
+const YggdrasilError = require("./YggdrasilError")
+const SessionError = require("./SessionError")
 const logger = require("../modules/logger")
 
 class ValidationError extends DefaultError {
@@ -34,11 +35,23 @@ class ValidationError extends DefaultError {
 
     serialize() {
         if (this.config.errorFormat === "YggdrasilError") {
-            return {
-                error: this.config.errorName || "IllegalArgumentException",
-                errorMessage: this.message,
-                cause: JSON.stringify(this.formattedErrors)
-            }
+            const err = new YggdrasilError(
+                this.code,
+                this.config.errorName || "IllegalArgumentException",
+                this.message,
+                JSON.stringify(this.formattedErrors)
+            )
+            return err.serialize()
+        }
+
+        if (this.config.errorFormat === "SessionError") {
+            const err = new SessionError(
+                this.code,
+                this.config.errorName || "Forbidden",
+                this.message,
+                this.context.path || ""
+            )
+            return err.serialize()
         }
 
         return {