.Net: Replace in-repo MEVD.Abstractions and ConformanceTests with NuGet package references

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Summary

Replaces the in-repo Microsoft.Extensions.VectorData.Abstractions and Microsoft.Extensions.VectorData.ConformanceTests projects with NuGet package references to the published 10.5.0 packages on nuget.org.

Changes

Package references

• Replace ProjectReference to in-repo MEVD.Abstractions and ConformanceTests with PackageReference to published 10.5.0 NuGet packages
• Centralize MEVD.Abstractions PackageReference in src/VectorData/Directory.Build.props
• Centralize ConformanceTests PackageReference in test/VectorData/Directory.Build.props
• Convert SemanticKernel.Abstractions ProjectReference to PackageReference

Polyfills and compatibility

• Add Microsoft.Bcl.Memory polyfill to 9 providers needing System.Index on net462/netstandard2.0 (SqlServer, PgVector, SqliteVec, AzureAISearch, CosmosNoSql, Pinecone, Qdrant, Redis, Weaviate)
• Add SuppressTfmSupportBuildWarnings for MEVD net462 buildTransitive warnings

Version bumps

• Bump System.Text.Json to 10.0.6 (required by MEAI.Abstractions 10.5.0)
• Bump Microsoft.Extensions.Configuration* and Microsoft.Extensions.DependencyInjection to 10.0.6 (required by ConformanceTests 10.5.0)

API compatibility fixes

dimensions

• Fix MongoModelBuilder.ProcessProperty override signature (removed 3rd Type? parameter)
• Add where TRecord : class constraint to VectorStoreTextSearch and related extension methods (required by MEVD 10.5.0's IVectorSearchable<TRecord>)

Test infrastructure cleanup

• Replace ConditionalFact/ConditionalTheory with standard xunit Fact/Theory attributes
• Remove conditional test infrastructure (ITestCondition, DisableTests, etc.)

Solution file cleanup

• Remove stale VectorData.Abstractions entries from solution files
• Update MEVD.slnf, SK-dotnet.slnx, SK-release.slnf

Build verification

MEVD.slnf builds clean with 0 warnings and 0 errors.