Fix multipart body arrays #2874

Job	Run time
Integration Tests (pdm.minimal.lock)	31s
Integration Tests (pdm.lock)	30s
test_min_deps (macos-latest)	51s
test_min_deps (windows-latest)	1m 19s
test_min_deps (ubuntu-latest)	51s
test (3.10, ubuntu-latest)	1m 14s
test (3.11, macos-latest)	48s
test (3.9, ubuntu-latest)	1m 20s
test (3.10, macos-latest)	1m 13s
test (3.9, windows-latest)	1m 48s
test (3.9, macos-latest)	1m 4s
test (3.11, ubuntu-latest)	1m 16s
test (3.10, windows-latest)	1m 48s
test (3.12, windows-latest)	1m 51s
test (3.12, macos-latest)	46s
test (3.12, ubuntu-latest)	1m 27s
test (3.13, ubuntu-latest)	1m 24s
test (3.11, windows-latest)	1m 45s
test (3.13, macos-latest)	46s
test (3.13, windows-latest)	1m 59s
Combine & check coverage	13s
	24m 44s

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