test: Add integration test for consumer group rebalancing

[josedab]

Description

Test that consumer group rebalancing works correctly when consumers join/leave.

Task

• Add an integration test that starts 2 consumers in a group
• Verify partition assignment changes when one consumer disconnects
• Use the testcontainers setup for a real Streamline server

Helpful Context

• See existing integration tests in tests/
• Use tokio::test with the testcontainers setup