Fix vector store disposal by retrieving from DI container

[Copilot]

Addresses resource leak from disposable vector stores (InMemoryVectorStore, QdrantVectorStore) being created but never disposed in GetVectorCollection.

Changes

• Retrieved vector stores from DI container instead of creating new instances

  • VectorDBType.Memory: kernel.Services.GetRequiredService<VectorStore>()
  • VectorDBType.Qdrant: kernel.Services.GetRequiredService<VectorStore>()
  • VectorDBType.Redis: kernel.Services.GetRequiredService<VectorStore>()
  • Default case: kernel.Services.GetRequiredService<VectorStore>()

• Removed Redis using block that was incorrectly disposing the store before the collection could be used

• Removed unused IDatabase database variable

Rationale

Vector stores are already registered in DI via ConfigVectorStore (e.g., AddInMemoryVectorStore(), AddQdrantVectorStore()). Creating new instances bypassed DI lifecycle management, causing leaks. The DI container now handles disposal.

// Before: Creates undisposed instance
vectorStore = new InMemoryVectorStore();

// After: Retrieves DI-managed instance
vectorStore = kernel.Services.GetRequiredService<VectorStore>();

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[JeffreySu]

@copilot retry this task again

[JeffreySu]

@copilot retry this task again, because it was conflicted. base on the latest code from #160

[JeffreySu]

@copilot retry this task again, because it was conflicted. base on the latest code from #160