fix(streamable-http): wait for post_writer before yielding #2206

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	26s
checks / test (3.10, locked, ubuntu-latest)	43s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, locked, windows-latest)	3m 2s
checks / test (3.11, lowest-direct, windows-latest)	3m 5s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, windows-latest)	3m 54s
checks / test (3.11, locked, windows-latest)	3m 14s
checks / test (3.14, locked, windows-latest)	4m 30s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.12, locked, windows-latest)	3m 28s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.12, locked, ubuntu-latest)	1m 0s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.13, locked, windows-latest)	2m 22s
checks / test (3.13, lowest-direct, windows-latest)	2m 11s
checks / test (3.12, lowest-direct, windows-latest)	3m 24s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.14, locked, ubuntu-latest)	2m 22s
checks / test (3.14, lowest-direct, windows-latest)	2m 31s
all-green	5s
	43m 18s

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