tests: resolve TOCTOU port race conditions for streamable-HTTP/SSE tests #2319

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	11s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.10, lowest-direct, windows-latest)	2m 41s
checks / test (3.11, lowest-direct, ubuntu-latest)	57s
checks / test (3.11, lowest-direct, windows-latest)	3m 18s
checks / test (3.10, locked, windows-latest)	2m 37s
checks / test (3.13, lowest-direct, windows-latest)	2m 54s
checks / test (3.14, lowest-direct, windows-latest)	2m 2s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.14, locked, ubuntu-latest)	2m 29s
checks / test (3.14, locked, windows-latest)	3m 35s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.12, locked, windows-latest)	3m 19s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 15s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.13, locked, windows-latest)	2m 20s
checks / test (3.11, locked, windows-latest)	2m 24s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.11, locked, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, windows-latest)	3m 18s
all-green	5s
	40m 25s

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