Refactor UserMetadataUpdater tests, optimize batch logic

- Split UserMetadataUpdater tests into focused files: Basic, Insert, License, Manager, and Metadata, improving organization and maintainability.
- Added UserMetadataUpdaterMetadataTests covering metadata enrichment, nullification, whitespace trimming, and lookup entity reuse.
- Improved batching performance in user/manager/license processing by replacing .Skip().Take() with .GetRange().
- Optimized manager resolution with O(1) AAD ID lookups and ensured tracked entities are used to prevent duplicate key errors.
- Enhanced handling of null/cleared metadata fields and duplicate UPNs.
- Updated .csproj to include new test files and removed old test region from UserImportTests.cs.

Author: sambetts

src/AnalyticsEngine/Tests.UnitTests/Tests.UnitTests.csproj

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\WebApplications\Microsoft.WebApplication.targets" />
   <PropertyGroup>
@@ -30,6 +30,11 @@
   </Target>
   <ItemGroup>
     <Compile Remove="Generated\**" />
+    <Compile Include="UserMetadataUpdaterBasicTests.cs" />
+    <Compile Include="UserMetadataUpdaterInsertTests.cs" />
+    <Compile Include="UserMetadataUpdaterLicenseTests.cs" />
+    <Compile Include="UserMetadataUpdaterManagerTests.cs" />
+    <Compile Include="UserMetadataUpdaterMetadataTests.cs" />
     <Content Include="Generated\**" />
   </ItemGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">