Add comprehensive Unicode tests for streamable HTTP transport #2770

Job	Run time
checks / pre-commit	37s
checks / readme-snippets	13s
checks / test (3.10, lowest-direct, ubuntu-latest)	36s
checks / test (3.11, lowest-direct, windows-latest)	1m 17s
checks / test (3.10, lowest-direct, windows-latest)	1m 18s
checks / test (3.10, highest, ubuntu-latest)	36s
checks / test (3.10, highest, windows-latest)	1m 24s
checks / test (3.11, highest, windows-latest)	1m 40s
checks / test (3.12, lowest-direct, ubuntu-latest)	33s
checks / test (3.11, lowest-direct, ubuntu-latest)	35s
checks / test (3.11, highest, ubuntu-latest)	34s
checks / test (3.13, lowest-direct, ubuntu-latest)	37s
checks / test (3.12, lowest-direct, windows-latest)	1m 23s
checks / test (3.13, highest, windows-latest)	1m 18s
checks / test (3.12, highest, ubuntu-latest)	35s
checks / test (3.13, lowest-direct, windows-latest)	1m 27s
checks / test (3.13, highest, ubuntu-latest)	34s
checks / test (3.12, highest, windows-latest)	1m 25s
	16m 42s

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