Retry MaxErrorRateExceeded under concurrent segment appends

[lyndonbauto]

Summary

• Retry MaxErrorRateExceeded alongside DeviceOverload in segment write and batch_write paths
• Double overload retry budget (5 → 10 attempts) for sustained back-pressure
• Set default client max_error_rate / error_rate_window in make_client() so stress workloads on a single CE node do not trip the client circuit breaker immediately

Context

Heavy concurrent append_event load (e.g. 64 writers on one session) against Docker CE can briefly return DeviceOverload, which cascades into MaxErrorRateExceeded with the stock client defaults. Segment rollover itself is fine — this is client/back-pressure resilience.

Test plan

• pytest tests/test_sessions.py::test_concurrent_append_flushes_under_write_block_size
• pytest full suite