tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1512

Job	Run time
checks / pre-commit	30s
checks / readme-snippets	14s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
checks / test (3.10, lowest-direct, ubuntu-latest)	53s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.10, locked, ubuntu-latest)	45s
checks / test (3.12, lowest-direct, windows-latest)	2m 24s
checks / test (3.10, locked, windows-latest)	1m 34s
checks / test (3.13, lowest-direct, windows-latest)	1m 47s
checks / test (3.10, lowest-direct, windows-latest)	1m 47s
checks / test (3.14, lowest-direct, ubuntu-latest)	59s
checks / test (3.11, locked, windows-latest)	1m 41s
checks / test (3.14, lowest-direct, windows-latest)	2m 0s
checks / test (3.13, locked, ubuntu-latest)	44s
checks / test (3.12, locked, windows-latest)	2m 12s
checks / test (3.13, locked, windows-latest)	1m 53s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.14, locked, ubuntu-latest)	1m 58s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
checks / test (3.14, locked, windows-latest)	3m 34s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.11, lowest-direct, windows-latest)	2m 12s
all-green	4s
	31m 33s

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