fix: fail fast when session is not started - #2631

Follow-up for CI: Python 3.14 was still failing branch coverage due to a missing arc reported on the pytest.raises(...) context manager line in ests/client/test_resource_cleanup.py. Added # pragma: no branch there to make branch coverage stable across 3.14. Pushed commit abbbc01.

Thanks for the PR, and sorry it sat here without a proper review. This has since landed via #2838.

We're closing most of the open PR backlog. v2 is out and changed a lot of the SDK, so many older PRs no longer apply as written, and we're a small team that realistically doesn't have the capacity to work through the rest.

If this still matters to you on v2, the most useful thing you can do is open an issue (or comment on the existing one) with your use case and a repro. Hearing why it matters to you is what we use to decide what to prioritise.

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