tests: resolve TOCTOU port race conditions for streamable-HTTP/SSE tests #2368

Job	Run time
checks / pre-commit	35s
checks / readme-snippets	10s
checks / test (3.11, locked, windows-latest)	2m 45s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 6s
checks / test (3.12, locked, windows-latest)	4m 15s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.10, locked, windows-latest)	3m 3s
checks / test (3.10, lowest-direct, windows-latest)	2m 45s
checks / test (3.11, locked, ubuntu-latest)	57s
checks / test (3.12, lowest-direct, windows-latest)	3m 26s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 20s
checks / test (3.11, lowest-direct, windows-latest)	2m 47s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 24s
checks / test (3.10, locked, ubuntu-latest)	1m 0s
checks / test (3.12, locked, ubuntu-latest)	1m 11s
checks / test (3.13, locked, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, windows-latest)	1m 49s
checks / test (3.14, locked, ubuntu-latest)	2m 30s
checks / test (3.13, locked, windows-latest)	2m 51s
checks / test (3.14, locked, windows-latest)	3m 53s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 10s
checks / test (3.14, lowest-direct, windows-latest)	2m 40s
all-green	5s
	43m 32s

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