tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1519

Job	Run time
checks / readme-snippets	12s
checks / pre-commit	35s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.12, lowest-direct, ubuntu-latest)	59s
checks / test (3.10, locked, windows-latest)	1m 43s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.11, lowest-direct, windows-latest)	1m 49s
checks / test (3.14, lowest-direct, windows-latest)	1m 57s
checks / test (3.11, locked, windows-latest)	1m 57s
checks / test (3.10, lowest-direct, windows-latest)	1m 53s
checks / test (3.12, locked, windows-latest)	2m 12s
checks / test (3.13, lowest-direct, windows-latest)	2m 1s
checks / test (3.12, locked, ubuntu-latest)	49s
checks / test (3.14, locked, windows-latest)	2m 35s
checks / test (3.12, lowest-direct, windows-latest)	2m 32s
checks / test (3.14, lowest-direct, ubuntu-latest)	54s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.11, lowest-direct, ubuntu-latest)	48s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.14, locked, ubuntu-latest)	2m 1s
checks / test (3.10, lowest-direct, ubuntu-latest)	52s
checks / test (3.13, locked, windows-latest)	1m 49s
all-green	5s
	31m 2s

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