Fix AsyncClient ownership and add WebChannel tests

[dennisvink]

This PR resolves #1 and #2.

Changes:

• track httpx.AsyncClient ownership in WebChannel
• only close internally-created clients in close()
• add pytest coverage for handshake, message dispatch, post-response ack/proxy stats, and shutdown ownership behavior
• add optional test dependency in pyproject.toml

This keeps caller-owned clients alive while adding regression coverage around the affected lifecycle and protocol paths.