fix(streamable-http): wait for post_writer before yielding #2543

Job	Run time
checks / pre-commit	39s
checks / readme-snippets	15s
checks / test (3.10, lowest-direct, windows-latest)	1m 17s
checks / test (3.11, lowest-direct, windows-latest)	1m 24s
checks / test (3.10, locked, windows-latest)	1m 52s
checks / test (3.11, lowest-direct, ubuntu-latest)	55s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.14, locked, ubuntu-latest)	1m 55s
checks / test (3.12, locked, ubuntu-latest)	51s
checks / test (3.12, lowest-direct, windows-latest)	1m 36s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.13, lowest-direct, windows-latest)	1m 27s
checks / test (3.13, locked, ubuntu-latest)	44s
checks / test (3.14, lowest-direct, windows-latest)	1m 35s
checks / test (3.14, lowest-direct, ubuntu-latest)	48s
checks / test (3.14, locked, windows-latest)	1m 53s
checks / test (3.13, locked, windows-latest)	1m 24s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.11, locked, windows-latest)	1m 22s
checks / test (3.12, locked, windows-latest)	1m 18s
checks / test (3.12, lowest-direct, ubuntu-latest)	59s
all-green	3s
	25m 40s

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