tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1525

Job	Run time
checks / pre-commit	39s
checks / readme-snippets	11s
checks / test (3.10, lowest-direct, windows-latest)	1m 14s
checks / test (3.10, locked, windows-latest)	1m 21s
checks / test (3.13, lowest-direct, ubuntu-latest)	52s
checks / test (3.11, lowest-direct, windows-latest)	1m 31s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.12, lowest-direct, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, windows-latest)	1m 31s
checks / test (3.11, locked, windows-latest)	1m 26s
checks / test (3.12, lowest-direct, windows-latest)	1m 15s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.14, locked, ubuntu-latest)	1m 48s
checks / test (3.14, lowest-direct, ubuntu-latest)	52s
checks / test (3.12, locked, windows-latest)	1m 26s
checks / test (3.13, locked, windows-latest)	1m 37s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
checks / test (3.14, lowest-direct, windows-latest)	1m 34s
checks / test (3.13, locked, ubuntu-latest)	41s
checks / test (3.14, locked, windows-latest)	2m 10s
all-green	4s
	25m 18s

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