Fix leaked anyio streams in streamable_http #587

Job	Run time
checks / pre-commit	35s
checks / readme-snippets	10s
checks / test (3.10, locked, ubuntu-latest)	52s
checks / test (3.10, lowest-direct, windows-latest)	2m 52s
checks / test (3.11, lowest-direct, windows-latest)	3m 8s
checks / test (3.12, locked, windows-latest)	3m 28s
checks / test (3.12, lowest-direct, windows-latest)	3m 44s
checks / test (3.10, locked, windows-latest)	2m 17s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, locked, ubuntu-latest)	2m 17s
checks / test (3.11, locked, ubuntu-latest)	48s
checks / test (3.13, locked, windows-latest)	2m 15s
checks / test (3.10, lowest-direct, ubuntu-latest)	59s
checks / test (3.14, lowest-direct, windows-latest)	3m 20s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.14, locked, windows-latest)	3m 7s
checks / test (3.13, lowest-direct, windows-latest)	2m 37s
checks / test (3.11, locked, windows-latest)	2m 46s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 19s
checks / test (3.13, locked, ubuntu-latest)	50s
checks / test (3.11, lowest-direct, ubuntu-latest)	57s
all-green	2s
	41m 6s

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