feat: refactor external input handling code

- refactor import/export code
- make import/export use the same JSON parsing as API
- make Patch.AssertIsValid actually useful

PluralKit.Core/Models/Patch/PatchObject.cs

@@ -1,17 +1,48 @@
 using System;
+using System.Text.RegularExpressions;
 
 namespace PluralKit.Core
 {
-
-    public class InvalidPatchException : Exception
-    {
-        public InvalidPatchException(string message) : base(message) {}
-    }
-
     public abstract class PatchObject
     {
         public abstract UpdateQueryBuilder Apply(UpdateQueryBuilder b);
 
-        public void CheckIsValid() {}
+        public void AssertIsValid() {}
+
+        protected bool AssertValid(string input, string name, int maxLength, Func<string, bool>? validate = null)
+        {
+            if (input.Length > maxLength)
+                throw new FieldTooLongError(name, maxLength, input.Length);
+            if (validate != null && !validate(input))
+                throw new ValidationError(name);
+            return true;
+        }
+
+        protected bool AssertValid(string input, string name, string pattern)
+        {
+            if (!Regex.IsMatch(input, pattern))
+                throw new ValidationError(name);
+            return true;
+        }
+    }
+
+    public class ValidationError: Exception
+    {
+        public ValidationError(string message): base(message) { }
+    }
+
+    public class FieldTooLongError: ValidationError
+    {
+        public string Name;
+        public int MaxLength;
+        public int ActualLength;
+
+        public FieldTooLongError(string name, int maxLength, int actualLength):
+            base($"{name} too long ({actualLength} > {maxLength})")
+        {
+            Name = name;
+            MaxLength = maxLength;
+            ActualLength = actualLength;
+        }
     }
 }