When we batch sessions for the endSessions command, the BSON array we generate after the first batch does not have indexes starting from 0. This causes a server error. Per the spec, we ignore errors for endSessions and I don't think we have tests that cause more than 10000 sessions to be pooled so this is missing test coverage.

Writing a test in mongo/integration would be expensive because we'd need to create 10k sessions, but we can write a test in mongo to manually reduce the batch size and use command monitoring to make assertions.