tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1444

Job	Run time
checks / pre-commit	38s
checks / readme-snippets	14s
checks / test (3.10, locked, ubuntu-latest)	46s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.10, locked, windows-latest)	1m 37s
checks / test (3.11, lowest-direct, ubuntu-latest)	47s
checks / test (3.10, lowest-direct, windows-latest)	1m 51s
checks / test (3.10, lowest-direct, ubuntu-latest)	53s
checks / test (3.11, locked, ubuntu-latest)	43s
checks / test (3.11, locked, windows-latest)	1m 59s
checks / test (3.12, lowest-direct, windows-latest)	2m 27s
checks / test (3.11, lowest-direct, windows-latest)	1m 49s
checks / test (3.13, locked, windows-latest)	1m 40s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.14, locked, ubuntu-latest)	8m 21s
checks / test (3.14, locked, windows-latest)	9m 12s
checks / test (3.14, lowest-direct, windows-latest)	9m 26s
checks / test (3.14, lowest-direct, ubuntu-latest)	7m 9s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.12, locked, windows-latest)	2m 32s
checks / test (3.13, lowest-direct, windows-latest)	2m 12s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
all-green	5s
	57m 45s

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