Fix multipart body arrays #2860

Job	Run time
test_min_deps (windows-latest)	1m 58s
test_min_deps (macos-latest)	1m 5s
test_min_deps (ubuntu-latest)	56s
Integration Tests (pdm.lock)	29s
Integration Tests (pdm.minimal.lock)	31s
test (3.11, macos-latest)	58s
test (3.9, macos-latest)	1m 7s
test (3.10, macos-latest)	1m 22s
test (3.9, ubuntu-latest)	1m 18s
test (3.10, windows-latest)	1m 54s
test (3.12, macos-latest)	1m 0s
test (3.12, windows-latest)	1m 58s
test (3.11, ubuntu-latest)	1m 15s
test (3.12, ubuntu-latest)	1m 25s
test (3.13, windows-latest)	1m 54s
test (3.13, macos-latest)	56s
test (3.13, ubuntu-latest)	1m 25s
test (3.9, windows-latest)	2m 31s
test (3.10, ubuntu-latest)	1m 15s
test (3.11, windows-latest)	1m 53s
Combine & check coverage	13s
	27m 23s

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