Propagate contextvars.Context through anyio streams without modifying SessionMessage #1718

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	32s
checks / test (3.10, locked, windows-latest)	2m 17s
checks / test (3.10, lowest-direct, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, ubuntu-latest)	50s
checks / test (3.10, lowest-direct, windows-latest)	3m 34s
checks / test (3.11, lowest-direct, windows-latest)	2m 32s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 5s
checks / test (3.10, locked, ubuntu-latest)	45s
checks / test (3.12, locked, windows-latest)	3m 4s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.11, locked, windows-latest)	2m 51s
checks / test (3.14, lowest-direct, windows-latest)	2m 0s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.13, locked, ubuntu-latest)	41s
checks / test (3.13, locked, windows-latest)	1m 57s
checks / test (3.14, locked, windows-latest)	4m 13s
checks / test (3.12, lowest-direct, windows-latest)	3m 22s
checks / test (3.14, locked, ubuntu-latest)	2m 22s
checks / test (3.13, lowest-direct, ubuntu-latest)	47s
checks / test (3.13, lowest-direct, windows-latest)	2m 36s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
all-green	2s
	39m 8s

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