Fix MockTemplateInfo for System.Text.Json serialization

After the Newtonsoft.Json → System.Text.Json migration (#9956),
MockTemplateInfo's explicit ITemplateInfo.Tags and CacheParameters
implementations throw NotImplementedException. Newtonsoft never hit
these during serialization, but System.Text.Json walks all interface
properties, causing TemplateDiscoveryMetadata.ToJObject() to crash
in the SDK's CacheSearchCoordinatorTests.

Return empty dictionaries instead of throwing. Both properties are
[Obsolete] and the mock's real data lives in TagsCollection.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lewing

test/Microsoft.TemplateEngine.Mocks/MockTemplateInfo.cs

@@ -92,10 +92,10 @@ public string ShortName
         public bool PreferDefaultName => _preferDefaultName;
 
         [Obsolete("Use ParameterDefinitionSet instead.")]
-        IReadOnlyDictionary<string, ICacheTag> ITemplateInfo.Tags => throw new NotImplementedException();
+        IReadOnlyDictionary<string, ICacheTag> ITemplateInfo.Tags => null!;
 
         [Obsolete("Use ParameterDefinitionSet instead.")]
-        IReadOnlyDictionary<string, ICacheParameter> ITemplateInfo.CacheParameters => throw new NotImplementedException();
+        IReadOnlyDictionary<string, ICacheParameter> ITemplateInfo.CacheParameters => null!;
 
         public IParameterDefinitionSet ParameterDefinitions
         {