Fix multipart body arrays #2862

Job	Run time
Integration Tests (pdm.lock)	22s
test_min_deps (ubuntu-latest)	56s
Integration Tests (pdm.minimal.lock)	32s
test_min_deps (windows-latest)	1m 43s
test (3.9, macos-latest)	1m 23s
test (3.9, ubuntu-latest)	1m 13s
test (3.9, windows-latest)	1m 58s
test_min_deps (macos-latest)	1m 0s
test (3.10, ubuntu-latest)	1m 22s
test (3.10, macos-latest)	1m 19s
test (3.10, windows-latest)	1m 59s
test (3.11, ubuntu-latest)	1m 10s
test (3.11, windows-latest)	1m 40s
test (3.11, macos-latest)	41s
test (3.13, ubuntu-latest)	1m 22s
test (3.13, windows-latest)	2m 6s
test (3.12, macos-latest)	51s
test (3.12, ubuntu-latest)	1m 24s
test (3.12, windows-latest)	1m 46s
test (3.13, macos-latest)	1m 0s
Combine & check coverage	11s
	25m 58s

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