Propagate contextvars.Context through anyio streams without modifying SessionMessage #1468

Job	Run time
checks / pre-commit	34s
checks / readme-snippets	14s
checks / test (3.11, lowest-direct, windows-latest)	2m 49s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, windows-latest)	3m 59s
checks / test (3.10, lowest-direct, ubuntu-latest)	46s
checks / test (3.11, locked, windows-latest)	2m 26s
checks / test (3.11, lowest-direct, ubuntu-latest)	50s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.11, locked, ubuntu-latest)	45s
checks / test (3.10, lowest-direct, windows-latest)	2m 33s
checks / test (3.13, lowest-direct, windows-latest)	2m 34s
checks / test (3.10, locked, windows-latest)	2m 7s
checks / test (3.12, locked, ubuntu-latest)	49s
checks / test (3.14, locked, windows-latest)	4m 12s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.12, locked, windows-latest)	3m 32s
checks / test (3.13, locked, windows-latest)	2m 47s
checks / test (3.14, lowest-direct, windows-latest)	3m 1s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 9s
checks / test (3.14, locked, ubuntu-latest)	2m 21s
checks / test (3.13, locked, ubuntu-latest)	47s
all-green	4s
	40m 53s

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