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

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]