Graceful SSE drain on session manager shutdown #1375

Job	Run time
checks / pre-commit	34s
checks / readme-snippets	15s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.10, locked, ubuntu-latest)	56s
checks / test (3.10, locked, windows-latest)	2m 42s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.11, lowest-direct, windows-latest)	2m 46s
checks / test (3.11, locked, windows-latest)	2m 28s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 20s
checks / test (3.14, locked, ubuntu-latest)	2m 25s
checks / test (3.12, locked, windows-latest)	3m 35s
checks / test (3.13, locked, windows-latest)	2m 30s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, lowest-direct, windows-latest)	2m 55s
checks / test (3.14, locked, windows-latest)	3m 12s
checks / test (3.13, lowest-direct, windows-latest)	2m 40s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 4s
checks / test (3.13, locked, ubuntu-latest)	50s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.10, lowest-direct, windows-latest)	3m 7s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, windows-latest)	3m 47s
all-green	3s
	41m 35s

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