revize endpointu: cteni NullableProperty, filtr v UTC, attachments

Kontrola vsech 180 operaci proti falesnemu iDoklad API a round trip
331 modelu SDK v obou smerech.

- SdkNullablePropertyConverter doplnuje cteni NullableProperty<T>.
  Konvertor SDK umi jen zapis, takze PATCH s takovou polozkou koncil
  prazdnou 500 uz pri cteni tela. Tykalo se 29 modelu. Zapis zustava
  na konvertoru SDK, odchozi payload se nemeni.
- Datum uvnitr NullableProperty se normalizuje na UTC stejne jako
  zbytek serializace.
- ListModifiers parsuje datum ve filtru s AdjustToUniversal a
  AssumeUniversal. Filtr se zonou se drive posouval o offset.
- POST /attachments kontroluje FileName a FileBytes, SDK na ne sahalo
  bez kontroly na null a vracelo neosetrenou 500.
- ExceptionHandlingMiddleware ma posledni zachyt, zadny request uz
  nekonci prazdnou 500.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Infrastructure/SdkContractResolver.cs

@@ -5,18 +5,17 @@ using Newtonsoft.Json.Serialization;
 namespace Idoklad.Infrastructure;
 
 /// <summary>
-/// Makes iDoklad SDK models serializable. Some SDK converters, attached through
-/// <see cref="JsonConverterAttribute"/>, only support reading and their <c>WriteJson</c> throws
-/// <see cref="NotImplementedException"/>, which turns any response carrying such a model into a
-/// 500. This resolver replaces them with <see cref="SdkWriteBypassJsonConverter"/>: reading keeps
-/// using the SDK converter, writing falls back to the standard serialization. Everything else is
+/// Makes iDoklad SDK models usable in both directions. The SDK only ever serializes what it sends
+/// and deserializes what it receives, so several of its converters, attached through
+/// <see cref="JsonConverterAttribute"/>, implement one direction and throw
+/// <see cref="NotImplementedException"/> in the other. This service needs both directions, so this
+/// resolver swaps those converters for wrappers that fill in the missing half. Everything else is
 /// left to <see cref="DefaultContractResolver"/>.
 /// </summary>
 public sealed class SdkContractResolver : DefaultContractResolver
 {
     /// <summary>
-    /// SDK converters that cannot write. Converters that do implement writing (for example
-    /// <c>NullablePropertyJsonConverter</c>) are deliberately not listed here.
+    /// SDK converters that cannot write, so responses carrying such a model would fail.
     /// </summary>
     private static readonly HashSet<string> WriteUnsupportedConverterTypeNames = new(StringComparer.Ordinal)
     {
@@ -27,6 +26,11 @@ public sealed class SdkContractResolver : DefaultContractResolver
         "IdokladSdk.Serialization.NotificationJsonConverter",
     };
 
+    /// <summary>
+    /// SDK converters that cannot read, so a request body carrying such a member would fail.
+    /// </summary>
+    private const string NullablePropertyConverterTypeName = "IdokladSdk.Clients.NullablePropertyJsonConverter";
+
     protected override JsonContract CreateContract(Type objectType)
     {
         var contract = base.CreateContract(objectType);
@@ -49,8 +53,19 @@ public sealed class SdkContractResolver : DefaultContractResolver
     {
         var typeName = converter?.GetType().FullName;
 
-        return typeName is not null && WriteUnsupportedConverterTypeNames.Contains(typeName)
-            ? new SdkWriteBypassJsonConverter(converter!)
+        if (typeName is null)
+        {
+            return converter;
+        }
+
+        if (WriteUnsupportedConverterTypeNames.Contains(typeName))
+        {
+            return new SdkWriteBypassJsonConverter(converter!);
+        }
+
+        // Attached to NullableProperty<T> members of Patch models; writing works, reading throws.
+        return typeName == NullablePropertyConverterTypeName
+            ? new SdkNullablePropertyConverter(converter!)
             : converter;
     }
 }