Fix leaked anyio streams in streamable_http #603

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	10s
checks / test (3.10, lowest-direct, windows-latest)	2m 41s
checks / test (3.11, locked, windows-latest)	2m 26s
checks / test (3.11, lowest-direct, windows-latest)	2m 40s
checks / test (3.14, locked, ubuntu-latest)	2m 20s
checks / test (3.11, locked, ubuntu-latest)	48s
checks / test (3.13, locked, windows-latest)	2m 41s
checks / test (3.12, lowest-direct, windows-latest)	4m 14s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 17s
checks / test (3.12, locked, windows-latest)	4m 0s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.13, lowest-direct, windows-latest)	2m 31s
checks / test (3.14, lowest-direct, windows-latest)	2m 39s
checks / test (3.11, lowest-direct, ubuntu-latest)	44s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, ubuntu-latest)	58s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, locked, windows-latest)	2m 19s
checks / test (3.10, locked, ubuntu-latest)	52s
checks / test (3.14, locked, windows-latest)	3m 15s
all-green	4s
	40m 43s

Provide feedback