Propagate contextvars.Context through anyio streams without modifying SessionMessage #1471

Job	Run time
checks / readme-snippets	12s
checks / pre-commit	32s
checks / test (3.11, locked, windows-latest)	2m 21s
checks / test (3.10, locked, ubuntu-latest)	59s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.10, lowest-direct, windows-latest)	2m 38s
checks / test (3.10, locked, windows-latest)	2m 14s
checks / test (3.13, lowest-direct, windows-latest)	2m 36s
checks / test (3.11, lowest-direct, windows-latest)	2m 49s
checks / test (3.11, locked, ubuntu-latest)	53s
checks / test (3.13, locked, windows-latest)	3m 9s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.14, locked, ubuntu-latest)	2m 22s
checks / test (3.12, lowest-direct, windows-latest)	3m 54s
checks / test (3.14, lowest-direct, windows-latest)	2m 46s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.12, locked, ubuntu-latest)	1m 0s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 19s
checks / test (3.14, locked, windows-latest)	3m 12s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.13, locked, ubuntu-latest)	47s
checks / test (3.12, locked, windows-latest)	3m 28s
all-green	2s
	41m 8s

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