shutdown CPU busy-loop in HTTP/SSE transports #915

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	14s
checks / test (3.11, lowest-direct, windows-latest)	3m 0s
checks / test (3.10, locked, ubuntu-latest)	52s
checks / test (3.10, lowest-direct, windows-latest)	2m 29s
checks / test (3.11, locked, ubuntu-latest)	51s
checks / test (3.13, lowest-direct, windows-latest)	2m 55s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.10, locked, windows-latest)	2m 43s
checks / test (3.12, lowest-direct, windows-latest)	3m 39s
checks / test (3.12, locked, windows-latest)	3m 52s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.13, lowest-direct, ubuntu-latest)	52s
checks / test (3.14, locked, windows-latest)	3m 48s
checks / test (3.12, lowest-direct, ubuntu-latest)	56s
checks / test (3.11, locked, windows-latest)	2m 34s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 11s
checks / test (3.13, locked, windows-latest)	3m 16s
checks / test (3.14, lowest-direct, windows-latest)	2m 28s
checks / test (3.14, locked, ubuntu-latest)	2m 30s
checks / test (3.11, lowest-direct, ubuntu-latest)	49s
all-green	4s
	42m 14s

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