fix: Chat Failure (#901)

## Description

Method 'ValidateKeyProperty' in type
'Microsoft.SemanticKernel.Connectors.PgVector.PostgresModelBuilder' from
assembly 'Microsoft.SemanticKernel.Connectors.PgVector,
Version=1.70.0.0, Culture=neutral, PublicKeyToken=f300afd708cefcd3' does
not have an implementation.

### Ensure that your pull request has followed all the steps below:
- [ ] Code compilation
- [ ] Created tests which fail without the change (if possible)
- [ ] All tests passing
- [ ] Extended the README / documentation, if necessary

Author: BenjaminMichaelis

.gitignore

@@ -6,6 +6,7 @@ EssentialCSharpManuscript.[0-9]
 .vs/
 obj
 bin
+TestResults/
 ~$*.do[ct]x
 ~*.tmp
 ~$*.dotm

Directory.Packages.props

@@ -12,6 +12,9 @@
       https://pkgs.dev.azure.com/intelliTect/_packaging/EssentialCSharp/nuget/v3/index.json;
     </RestoreSources>
   </PropertyGroup>
+  <PropertyGroup>
+    <SemanticKernelVersion>1.72.0</SemanticKernelVersion>
+  </PropertyGroup>
   <ItemGroup Condition="$(AccessToNugetFeed)">
     <PackageVersion Include="ContentFeedNuget" Version="$(ToolingPackagesVersion)" />
   </ItemGroup>
@@ -36,8 +39,8 @@
     <PackageVersion Include="Microsoft.EntityFrameworkCore.Sqlite" Version="10.0.3" />
     <PackageVersion Include="Microsoft.EntityFrameworkCore.SqlServer" Version="10.0.3" />
     <PackageVersion Include="Microsoft.EntityFrameworkCore.Tools" Version="10.0.3" />
-    <PackageVersion Include="Microsoft.SemanticKernel" Version="1.72.0" />
-    <PackageVersion Include="Microsoft.SemanticKernel.Connectors.PgVector" Version="1.70.0-preview" />
+    <PackageVersion Include="Microsoft.SemanticKernel" Version="$(SemanticKernelVersion)" />
+    <PackageVersion Include="Microsoft.SemanticKernel.Connectors.PgVector" Version="$(SemanticKernelVersion)-preview" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="10.0.103" />
     <PackageVersion Include="Microsoft.VisualStudio.Azure.Containers.Tools.Targets" Version="1.23.0" />
     <PackageVersion Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="10.0.2" />

EssentialCSharp.Chat.Tests/PgVectorConnectorTests.cs

@@ -0,0 +1,28 @@
+using EssentialCSharp.Chat.Common.Models;
+using Microsoft.SemanticKernel.Connectors.PgVector;
+
+namespace EssentialCSharp.Chat.Tests;
+
+public class PgVectorConnectorTests
+{
+    /// <summary>
+    /// Verifies that PostgresVectorStore.GetCollection does not throw a TypeLoadException,
+    /// which would indicate a version mismatch between Microsoft.SemanticKernel core and
+    /// Microsoft.SemanticKernel.Connectors.PgVector (e.g., missing vtable slots on
+    /// internal types like PostgresModelBuilder).
+    /// </summary>
+    [Test]
+    public async Task GetCollection_WithBookContentChunk_DoesNotThrowTypeLoadException()
+    {
+        // Arrange — no real DB connection is needed; connections are only opened for actual queries
+#pragma warning disable SKEXP0010 // PostgresVectorStore is experimental
+        using var store = new PostgresVectorStore("Host=localhost;Database=test;Username=test;Password=test");
+
+        // Act — this triggers loading internal PostgresModelBuilder via PostgresCollection ctor
+        var collection = store.GetCollection<string, BookContentChunk>("test-collection");
+#pragma warning restore SKEXP0010
+
+        // Assert
+        await Assert.That(collection).IsNotNull();
+    }
+}