tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1637

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	15s
checks / test (3.10, locked, ubuntu-latest)	48s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, lowest-direct, ubuntu-latest)	55s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.11, locked, windows-latest)	1m 21s
checks / test (3.14, locked, windows-latest)	2m 18s
checks / test (3.12, locked, ubuntu-latest)	51s
checks / test (3.10, lowest-direct, windows-latest)	1m 15s
checks / test (3.14, locked, ubuntu-latest)	1m 55s
checks / test (3.14, lowest-direct, windows-latest)	1m 16s
checks / test (3.11, lowest-direct, ubuntu-latest)	54s
checks / test (3.12, locked, windows-latest)	1m 18s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.13, lowest-direct, windows-latest)	1m 29s
checks / test (3.11, lowest-direct, windows-latest)	1m 30s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.14, lowest-direct, ubuntu-latest)	49s
checks / test (3.10, locked, windows-latest)	1m 12s
checks / test (3.13, locked, windows-latest)	1m 27s
checks / test (3.12, lowest-direct, windows-latest)	1m 31s
all-green	10s
	25m 6s

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