fix: fail fast when session is not started #2157

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	37s
checks / test (3.10, lowest-direct, ubuntu-latest)	49s
checks / test (3.13, locked, windows-latest)	2m 21s
checks / test (3.11, locked, ubuntu-latest)	44s
checks / test (3.12, lowest-direct, windows-latest)	3m 31s
checks / test (3.10, lowest-direct, windows-latest)	2m 48s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.11, lowest-direct, windows-latest)	3m 30s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.12, locked, windows-latest)	3m 23s
checks / test (3.14, locked, windows-latest)	3m 48s
checks / test (3.10, locked, windows-latest)	1m 59s
checks / test (3.14, lowest-direct, windows-latest)	2m 18s
checks / test (3.11, locked, windows-latest)	3m 17s
checks / test (3.13, lowest-direct, ubuntu-latest)	44s
checks / test (3.14, locked, ubuntu-latest)	2m 19s
checks / test (3.11, lowest-direct, ubuntu-latest)	50s
checks / test (3.13, locked, ubuntu-latest)	43s
checks / test (3.12, lowest-direct, ubuntu-latest)	54s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.13, lowest-direct, windows-latest)	3m 10s
all-green	5s
	40m 54s

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