Phase 3 complete: Add HTTP-level REST API integration tests (23 new tests)

Co-authored-by: devstress <30769729+devstress@users.noreply.github.com>

Author: Copilot

FlinkDotNet/FlinkDotNet.JobManager.Tests/FlinkDotNet.JobManager.Tests.csproj

@@ -10,6 +10,7 @@
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.2" />
     <PackageReference Include="FluentAssertions" Version="8.8.0" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="9.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="xunit" Version="2.9.2" />