tests: eliminate port-allocation races in SSE/StreamableHTTP tests #1405

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	12s
checks / test (3.10, locked, windows-latest)	1m 56s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.11, locked, ubuntu-latest)	44s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 5s
checks / test (3.10, lowest-direct, windows-latest)	2m 0s
checks / test (3.14, lowest-direct, ubuntu-latest)	7m 32s
checks / test (3.12, locked, ubuntu-latest)	57s
checks / test (3.13, lowest-direct, windows-latest)	2m 0s
checks / test (3.11, lowest-direct, windows-latest)	2m 4s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.12, locked, windows-latest)	2m 30s
checks / test (3.14, locked, windows-latest)	9m 28s
checks / test (3.14, locked, ubuntu-latest)	8m 39s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.13, locked, windows-latest)	2m 0s
checks / test (3.11, lowest-direct, ubuntu-latest)	49s
checks / test (3.14, lowest-direct, windows-latest)	8m 4s
checks / test (3.12, lowest-direct, windows-latest)	2m 30s
checks / test (3.11, locked, windows-latest)	1m 57s
all-green	5s
	58m 25s

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