Wrap disposable vector stores in using statements to prevent resource leaks

[Copilot]

Code analysis flagged that InMemoryVectorStore and QdrantVectorStore instances were created but never disposed, causing resource leaks.

Changes

• Wrapped InMemoryVectorStore creation in Memory and default cases with using statements
• Wrapped QdrantVectorStore creation with using statement
• Removed unused vectorStore variable declaration

This follows the pattern already established for RedisVectorStore in commit bb6eda4, ensuring consistent resource management across all vector store types.

// Before
vectorStore = new InMemoryVectorStore();
collection = vectorStore.GetCollection<TKey, TRecord>(name, vectorStoreRecordDefinition);

// After
using (var inMemoryVectorStore = new InMemoryVectorStore())
{
    collection = inMemoryVectorStore.GetCollection<TKey, TRecord>(name, vectorStoreRecordDefinition);
}

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