fix(streamable-http): wait for post_writer before yielding #2208

Job	Run time
checks / pre-commit	35s
checks / readme-snippets	10s
checks / test (3.11, lowest-direct, windows-latest)	2m 27s
checks / test (3.10, lowest-direct, windows-latest)	2m 37s
checks / test (3.10, locked, ubuntu-latest)	56s
checks / test (3.12, lowest-direct, windows-latest)	3m 34s
checks / test (3.11, lowest-direct, ubuntu-latest)	59s
checks / test (3.12, locked, windows-latest)	3m 44s
checks / test (3.10, locked, windows-latest)	2m 24s
checks / test (3.11, locked, ubuntu-latest)	51s
checks / test (3.11, locked, windows-latest)	2m 35s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.12, locked, ubuntu-latest)	1m 1s
checks / test (3.14, locked, windows-latest)	3m 26s
checks / test (3.13, lowest-direct, windows-latest)	2m 54s
checks / test (3.14, locked, ubuntu-latest)	2m 24s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 23s
checks / test (3.13, locked, windows-latest)	2m 40s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, lowest-direct, windows-latest)	2m 28s
checks / test (3.13, locked, ubuntu-latest)	51s
all-green	5s
	41m 3s

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