feat: add connect method to verify gRPC connectivity

[Maki-Grz]

This PR resolves issue #258 where the Qdrant client was successfully built even if the server was unreachable due to the "lazy" nature of gRPC channels.
Changes:

• Added connect() method in ChannelPool to explicitly trigger make_channel() and the underlying Tonic connection.
• Exposed pub async fn connect(&self) in the Qdrant client for users to eagerly verify connectivity.
• Added an integration test snippet in tests/snippets/connect.rs to verify that connect() correctly returns an error on invalid URLs.

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