Propagate contextvars.Context through anyio streams without modifying SessionMessage #1470

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	12s
checks / test (3.10, lowest-direct, windows-latest)	2m 37s
checks / test (3.10, locked, ubuntu-latest)	53s
checks / test (3.10, locked, windows-latest)	2m 22s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.14, locked, ubuntu-latest)	2m 19s
checks / test (3.13, lowest-direct, windows-latest)	2m 53s
checks / test (3.12, locked, ubuntu-latest)	1m 0s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.11, locked, windows-latest)	2m 40s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 19s
checks / test (3.14, locked, windows-latest)	3m 8s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.14, lowest-direct, windows-latest)	2m 25s
checks / test (3.12, lowest-direct, windows-latest)	4m 7s
checks / test (3.13, lowest-direct, ubuntu-latest)	52s
checks / test (3.13, locked, windows-latest)	2m 25s
checks / test (3.11, lowest-direct, windows-latest)	2m 54s
checks / test (3.12, locked, windows-latest)	4m 13s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
all-green	5s
	41m 23s

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