Fix multipart body arrays #2857

Job	Run time
test_min_deps (macos-latest)	53s
test_min_deps (windows-latest)	1m 58s
Integration Tests (pdm.minimal.lock)	7s
test_min_deps (ubuntu-latest)	58s
Integration Tests (pdm.lock)	9s
test (3.9, macos-latest)	1m 6s
test (3.9, ubuntu-latest)	1m 19s
test (3.10, macos-latest)	1m 22s
test (3.10, ubuntu-latest)	1m 14s
test (3.11, windows-latest)	2m 6s
test (3.10, windows-latest)	1m 46s
test (3.9, windows-latest)	2m 0s
test (3.11, macos-latest)	46s
test (3.12, ubuntu-latest)	1m 29s
test (3.13, ubuntu-latest)	1m 30s
test (3.13, macos-latest)	49s
test (3.11, ubuntu-latest)	1m 20s
test (3.12, windows-latest)	2m 12s
test (3.13, windows-latest)	1m 45s
test (3.12, macos-latest)	47s
Combine & check coverage	13s
	25m 49s

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