.Net: Update SK to use MEVD packages and move MEVD projects out of main solution

[westey-m]

Motivation and Context

• The MEVD packages are not shipped from this repo anymore.

Description

• Moving the packages out of the main SK solution
• Update all projects to use MEVD.Abstractions package directly
• Revert some previous code changes to make the MEVD implementations compatible with the last MEVD.Abstractions package released from this repo.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Pull request overview

Updates the .NET solution/project structure so SK consumes MEVD (VectorData connector) NuGet packages instead of building MEVD projects as part of the main SK solution, and introduces a dedicated MEVD solution for working on those projects separately.

Changes:

• Switched various SK tests/samples from src/VectorData/* ProjectReferences to Microsoft.SemanticKernel.Connectors.* PackageReferences.
• Removed VectorData projects/tests from dotnet/SK-dotnet.slnx and added a new dotnet/MEVD.slnx containing those projects.
• Added central package versions for Microsoft.Extensions.VectorData.Abstractions and the Microsoft.SemanticKernel.Connectors.* VectorData connector packages.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/SemanticKernel.UnitTests/SemanticKernel.UnitTests.csproj	Replaces VectorData InMemory project reference with the InMemory connector package.
dotnet/src/SemanticKernel.Abstractions/SemanticKernel.Abstractions.csproj	Replaces local VectorData abstractions project reference with Microsoft.Extensions.VectorData.Abstractions package.
dotnet/src/IntegrationTests/IntegrationTests.csproj	Removes VectorData connector project references (requires follow-up for missing InMemory dependency).
dotnet/SK-dotnet.slnx	Removes VectorData projects/tests from the main SK solution.
dotnet/samples/GettingStartedWithVectorStores/GettingStartedWithVectorStores.csproj	Updates vector store sample to use connector packages instead of VectorData project references.
dotnet/samples/GettingStartedWithTextSearch/GettingStartedWithTextSearch.csproj	Updates text search sample to use the InMemory connector package.
dotnet/samples/Demos/VectorStoreRAG/VectorStoreRAG.csproj	Updates RAG demo to use connector packages instead of VectorData project references.
dotnet/samples/Demos/OnnxSimpleRAG/OnnxSimpleRAG.csproj	Updates ONNX RAG demo to use the InMemory connector package.
dotnet/samples/Demos/ModelContextProtocolClientServer/MCPServer/MCPServer.csproj	Updates MCP server demo to use the InMemory connector package.
dotnet/samples/Demos/AgentFrameworkWithAspire/ChatWithAgent.ApiService/ChatWithAgent.ApiService.csproj	Adds AzureAISearch connector package (removes VectorData project reference).
dotnet/samples/Concepts/Memory/HuggingFace_TextEmbeddingCustomHttpHandler.cs	Updates the sample to use InMemoryCollection instead of SqliteCollection.
dotnet/samples/Concepts/Concepts.csproj	Adds connector package references (and explicit SharpCompress/Snappier refs) and removes VectorData project references.
dotnet/MEVD.slnx	New dedicated solution containing VectorData projects and their tests.
dotnet/Directory.Packages.props	Adds central versions for Microsoft.Extensions.VectorData.Abstractions and Microsoft.SemanticKernel.Connectors.* VectorData connector packages.

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