tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1511

Job	Run time
checks / pre-commit	37s
checks / readme-snippets	10s
checks / test (3.10, locked, ubuntu-latest)	46s
checks / test (3.11, lowest-direct, windows-latest)	2m 1s
checks / test (3.10, lowest-direct, windows-latest)	1m 34s
checks / test (3.11, locked, windows-latest)	1m 50s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.14, lowest-direct, windows-latest)	1m 53s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.13, locked, windows-latest)	1m 54s
checks / test (3.12, locked, ubuntu-latest)	50s
checks / test (3.10, locked, windows-latest)	1m 48s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.13, locked, ubuntu-latest)	42s
checks / test (3.13, lowest-direct, windows-latest)	2m 15s
checks / test (3.12, locked, windows-latest)	2m 27s
checks / test (3.14, lowest-direct, ubuntu-latest)	49s
checks / test (3.14, locked, ubuntu-latest)	2m 0s
checks / test (3.13, lowest-direct, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, ubuntu-latest)	58s
checks / test (3.12, lowest-direct, windows-latest)	2m 36s
checks / test (3.14, locked, windows-latest)	2m 49s
all-green	4s
	31m 24s

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