Fix multipart body arrays #2863

Job	Run time
Integration Tests (pdm.lock)	29s
Integration Tests (pdm.minimal.lock)	32s
test_min_deps (ubuntu-latest)	52s
test_min_deps (windows-latest)	1m 50s
test_min_deps (macos-latest)	58s
test (3.9, macos-latest)	1m 11s
test (3.10, ubuntu-latest)	1m 21s
test (3.11, macos-latest)	55s
test (3.10, macos-latest)	1m 16s
test (3.9, ubuntu-latest)	1m 18s
test (3.13, macos-latest)	1m 14s
test (3.9, windows-latest)	1m 52s
test (3.12, macos-latest)	53s
test (3.12, windows-latest)	2m 5s
test (3.12, ubuntu-latest)	1m 22s
test (3.11, windows-latest)	2m 7s
test (3.13, ubuntu-latest)	1m 26s
test (3.13, windows-latest)	2m 3s
test (3.10, windows-latest)	1m 44s
test (3.11, ubuntu-latest)	1m 17s
Combine & check coverage	16s
	27m 1s

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