Context

PYTHON-6167 makes the load-balancer Evergreen tasks run the whole default suite under loadBalanced=true. The first run failed 4 tests per task, and the PR now skips them in LB mode. Two underlying gaps:

• Pinned-cursor getMores never retry on SystemOverloadedError and RetryableError errors; the retryable reads and Client Backpressure specs expect retries in LB topology. Failing: test_retry_overload_error_getMore and the two TestClientBackpressureGetMoreRetried tests from test/client-backpressure/getMore-retried.json.
• test_retryable_writes_in_sharded_cluster_multiple_available builds a multi-mongos client, which is invalid with loadBalanced=true, and its premise (a retry on another mongos) does not apply in LB mode.

Evidence: Evergreen patch 6af4c3a24253440007a3fa1b, tasks test-load-balancer-v9.0-python3.14t-sync-auth-ssl-sharded-cluster-cov and test-load-balancer-v9.0-python3.14t-async-auth-ssl-sharded-cluster-cov. The suite already carries other LB-mode skips.

Definition of done

• The whole suite passes in the LB tasks with no LB-mode skips beyond documented ones.

Pitfalls

• getMore retries interact with cursor pinning and retryable-read session semantics.
• The retryable reads, load balancer, and Client Backpressure specs all constrain the behavior.