fix(streamable-http): wait for post_writer before yielding #2468

Job	Run time
checks / test (3.14, locked, ubuntu-latest)	1m 54s
checks / test (3.13, locked, windows-latest)	1m 10s
checks / test (3.10, locked, windows-latest)	1m 14s
checks / test (3.14, lowest-direct, windows-latest)	1m 14s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.14, locked, windows-latest)	1m 53s
checks / test (3.12, lowest-direct, windows-latest)	1m 26s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
checks / test (3.13, lowest-direct, windows-latest)	1m 19s
checks / test (3.14, lowest-direct, ubuntu-latest)	54s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.11, locked, windows-latest)	2m 11s
checks / pre-commit	31s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.10, locked, ubuntu-latest)	48s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.11, lowest-direct, windows-latest)	1m 24s
checks / test (3.10, lowest-direct, windows-latest)	1m 17s
checks / test (3.12, locked, windows-latest)	1m 21s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / readme-snippets	14s
all-green	3s
	24m 58s

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