tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1515

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	13s
checks / test (3.10, locked, ubuntu-latest)	46s
checks / test (3.10, locked, windows-latest)	1m 37s
checks / test (3.11, lowest-direct, ubuntu-latest)	47s
checks / test (3.12, lowest-direct, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, windows-latest)	1m 45s
checks / test (3.11, locked, ubuntu-latest)	48s
checks / test (3.12, locked, ubuntu-latest)	51s
checks / test (3.11, locked, windows-latest)	1m 45s
checks / test (3.10, lowest-direct, ubuntu-latest)	51s
checks / test (3.11, lowest-direct, windows-latest)	2m 11s
checks / test (3.12, lowest-direct, windows-latest)	2m 13s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.12, locked, windows-latest)	2m 10s
checks / test (3.14, locked, windows-latest)	2m 26s
checks / test (3.14, lowest-direct, ubuntu-latest)	58s
checks / test (3.13, locked, ubuntu-latest)	44s
checks / test (3.14, lowest-direct, windows-latest)	1m 58s
checks / test (3.13, locked, windows-latest)	1m 33s
checks / test (3.13, lowest-direct, windows-latest)	1m 52s
checks / test (3.14, locked, ubuntu-latest)	2m 0s
all-green	4s
	29m 50s

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