Bootstrap MEVD vector data providers

[roji]

Port MEVD (Microsoft Extensions Vector Data) provider implementations from the Semantic Kernel repository into CommunityToolkit.AI.

Providers (9 total)

AzureAISearch, CosmosMongoDB, InMemory, PgVector, Pinecone, Qdrant, Redis, SqliteVec, Weaviate

Infrastructure

• Namespace migration: Microsoft.SemanticKernel.Connectors.* → CommunityToolkit.VectorData.*
• Central package management via Directory.Packages.props
• Legacy TFM polyfills (Shared/LegacySupport/) for net462/netstandard2.0
• Shared utilities: Throw.cs, VectorStoreErrorHandler.cs
• global.json pinning .NET 10 SDK
• GitHub Actions CI workflow (.github/workflows/mevd.yml)

Test coverage

• Unit tests: All 9 providers (511 tests passing)
• Conformance tests: 7 providers run in CI via Testcontainers (InMemory, PgVector, Pinecone, Qdrant, Redis, SqliteVec, Weaviate)
• AzureAISearch and CosmosMongoDB conformance tests excluded from CI (require cloud instances), but verified locally

Notable fixes

• Workaround for CollectionModelBuilder.SetPropertyStorageName bug in MEVD Abstractions 10.5.0 (StorageName not respecting UsesExternalSerializer flag)
• CosmosMongo: Fixed filter overrides for unsupported operations (Any, Contains variants)
• CosmosMongo: DateTimeOffset filtering marked as unsupported (pre-existing mismatch between BsonSerializer and BsonValueFactory representations, also failing in SK)

[roji]

This moves all providers except for SQL Server, Cosmos NoSQL and MongoDB from the Semantic Kernel repo to here.

I made as few changes to product code as possible compared to the Semantic Kernel origin. On the test side I did do some extensive stabilization work, so that now all conformance tests are passing; all but Cosmos Mongo and Azure AI Search are now running in CI (most against Testcontainers) and seem quite reliable (those two exceptions have no Testcontainer option and require an actual cloud instance).

[roji]

@westey-m (and @jeffhandley) pushed a commit which ports microsoft/semantic-kernel#13902 to here.

[jeffhandley]

Thanks for your patience, @roji. This is super comprehensive and from what I can tell everything moved over cleanly. The dev environment is straightforward/idiomatic without challenging setup steps, and the code style is consistent throughout.

I didn't review any of the code for correctness; I trust that's all been reviewed/tested ahead of this transition.

[roji]

Thanks for spending time on this @jeffhandley! I've figure out publishing logistics for this next and we'll get the nugets out (previews).