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.

schemas/sessionsserver/session/minecraft/join.js

@@ -0,0 +1,22 @@
+const z = require("zod")
+
+module.exports = {
+    POST: {
+        body: z.object({
+            accessToken: z.string()
+                .min(1, { message: "Access Token is required." }),
+            selectedProfile: z.string()
+                .length(32, { message: "Selected Profile must be a valid UUID without dashes." })
+                .regex(/^[0-9a-fA-F]+$/, { message: "Selected Profile must be hexadecimal." }),
+            serverId: z.string()
+                .min(1, { message: "Server ID is required." })
+                .max(42, { message: "Server ID is too long." }) 
+        }),
+        error: {
+            code: 400,
+            message: "Missing or invalid parameters.",
+            errorFormat: "SessionError",
+            errorName: "Forbidden"
+        }
+    }
+}