fix(streamable-http): wait for post_writer before yielding #2207

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	11s
checks / test (3.10, locked, windows-latest)	2m 23s
checks / test (3.10, lowest-direct, windows-latest)	39s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.11, lowest-direct, windows-latest)	2m 28s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.10, locked, ubuntu-latest)	56s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, windows-latest)	3m 52s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.11, locked, windows-latest)	1m 39s
checks / test (3.13, lowest-direct, ubuntu-latest)	59s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.13, locked, ubuntu-latest)	51s
checks / test (3.14, locked, ubuntu-latest)	2m 39s
checks / test (3.14, locked, windows-latest)	3m 11s
checks / test (3.12, locked, windows-latest)	3m 28s
checks / test (3.14, lowest-direct, windows-latest)	2m 25s
checks / test (3.13, lowest-direct, windows-latest)	2m 27s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.13, locked, windows-latest)	2m 18s
all-green	3s
	37m 13s

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