tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1389

Job	Run time
checks / pre-commit	38s
checks / readme-snippets	12s
checks / test (3.12, lowest-direct, windows-latest)	2m 29s
checks / test (3.10, lowest-direct, windows-latest)	2m 13s
checks / test (3.11, lowest-direct, ubuntu-latest)	49s
checks / test (3.10, locked, windows-latest)	1m 37s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.12, locked, ubuntu-latest)	58s
checks / test (3.13, locked, windows-latest)	1m 36s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.11, locked, windows-latest)	1m 45s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.14, locked, ubuntu-latest)	8m 46s
checks / test (3.11, lowest-direct, windows-latest)	2m 5s
checks / test (3.13, lowest-direct, windows-latest)	1m 49s
checks / test (3.12, locked, windows-latest)	2m 13s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 4s
checks / test (3.14, locked, windows-latest)	8m 34s
checks / test (3.14, lowest-direct, ubuntu-latest)	7m 30s
checks / test (3.14, lowest-direct, windows-latest)	8m 6s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
all-green	3s
	56m 42s

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