NEO-48: omit null reasonCode from success JSON responses

WhenWritingNull on PerkBranchSelectResponse and SkillProgressionGrantResponse
matches hotbar/cast/interaction; fixes Bruno tier1 undefined check.

bruno/neon-sprawl-server/perk-state/Post branch select tier1.bru

@@ -6,6 +6,7 @@ meta {
 
 docs {
   NEO-48: tier-1 salvage branch pick. Run skill-progression grant (100+ XP for level 2) first if this denies level_too_low.
+  On success, reasonCode is omitted from JSON (not null); test expects undefined.
 }
 
 post {

server/NeonSprawl.Server.Tests/Game/Mastery/PerkStateApiTests.cs

@@ -2,6 +2,7 @@ using System.Linq;
 using System.Net;
 using System.Net.Http;
 using System.Net.Http.Json;
+using System.Text.Json;
 using System.Text;
 using NeonSprawl.Server.Game.Mastery;
 using NeonSprawl.Server.Game.Skills;
@@ -156,7 +157,13 @@ public sealed class PerkStateApiTests
 
         // Assert
         Assert.Equal(HttpStatusCode.OK, post.StatusCode);
-        var envelope = await post.Content.ReadFromJsonAsync<PerkBranchSelectResponse>();
+        var json = await post.Content.ReadAsStringAsync();
+        using (var doc = JsonDocument.Parse(json))
+        {
+            Assert.False(doc.RootElement.TryGetProperty("reasonCode", out _));
+        }
+
+        var envelope = JsonSerializer.Deserialize<PerkBranchSelectResponse>(json);
         Assert.NotNull(envelope);
         Assert.True(envelope!.Selected);
         Assert.Null(envelope.ReasonCode);

server/NeonSprawl.Server/Game/Mastery/PerkStateDtos.cs

@@ -69,6 +69,7 @@ public sealed class PerkBranchSelectResponse
     public bool Selected { get; init; }
 
     [JsonPropertyName("reasonCode")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public string? ReasonCode { get; init; }
 
     [JsonPropertyName("perkState")]

server/NeonSprawl.Server/Game/Skills/SkillProgressionGrantDtos.cs

@@ -34,6 +34,7 @@ public sealed class SkillProgressionGrantResponse
 
     /// <remarks>Set when <see cref="Granted"/> is <c>false</c>; stable snake_case values (see README).</remarks>
     [JsonPropertyName("reasonCode")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public string? ReasonCode { get; init; }
 
     [JsonPropertyName("progression")]