refactor(core): harden PluginManifest error model

Addresses code quality review of c082ecf:
- Require takes a literal JSON field name, no more fragile PascalCase->camelCase transform
- Parse_MissingRequiredField_Throws asserts exact message, not substring
- Remove unused using System.Text.Json.Serialization

src/AcDream.Core/Plugins/PluginManifest.cs

@@ -1,5 +1,4 @@
 using System.Text.Json;
-using System.Text.Json.Serialization;
 
 namespace AcDream.Core.Plugins;
 
@@ -26,10 +25,10 @@ public sealed record PluginManifest(
         if (dto is null)
             throw new PluginManifestException("manifest is empty");
 
-        Require(dto.Id, nameof(dto.Id));
-        Require(dto.DisplayName, nameof(dto.DisplayName));
-        Require(dto.Version, nameof(dto.Version));
-        Require(dto.EntryDll, nameof(dto.EntryDll));
+        Require(dto.Id, "id");
+        Require(dto.DisplayName, "displayName");
+        Require(dto.Version, "version");
+        Require(dto.EntryDll, "entryDll");
         if (dto.ApiVersion <= 0)
             throw new PluginManifestException("apiVersion must be >= 1");
 
@@ -42,11 +41,10 @@ public sealed record PluginManifest(
             dto.Dependencies ?? Array.Empty<string>());
     }
 
-    private static void Require(string? value, string fieldName)
+    private static void Require(string? value, string jsonFieldName)
     {
         if (string.IsNullOrWhiteSpace(value))
-            throw new PluginManifestException(
-                $"missing required field: {char.ToLowerInvariant(fieldName[0])}{fieldName[1..]}");
+            throw new PluginManifestException($"missing required field: {jsonFieldName}");
     }
 
     private static readonly JsonSerializerOptions JsonOptions = new()

tests/AcDream.Core.Tests/Plugins/PluginManifestTests.cs

@@ -34,7 +34,7 @@ public class PluginManifestTests
         """;
 
         var ex = Assert.Throws<PluginManifestException>(() => PluginManifest.Parse(json));
-        Assert.Contains("displayName", ex.Message);
+        Assert.Equal("missing required field: displayName", ex.Message);
     }
 
     [Fact]