Fix bug re `custom_init` on members of `Union` #3961

Job	Run time
coverage	2m 12s
test on ubuntu	2m 13s
test on macos	4m 40s
test 3.7	2m 11s
test on windows	5m 19s
test 3.8	2m 1s
test 3.9	2m 20s
rust benchmarks	2m 58s
test 3.10	2m 5s
lint	49s
test 3.11	2m 0s
test 3.12	2m 40s
test-debug 3.11	50s
test pypy3.7	2m 57s
test-debug pypy3.10	1m 16s
test pypy3.8	2m 30s
test-pydantic-integration	1m 30s
test pypy3.9	2m 32s
test pypy3.10	2m 31s
build sdist	10s
build-wasm-emscripten	2m 20s
build on ${{ matrix.os }} (${{ matrix.target }} - ${{ matrix.interpreter \|\| 'all' }}${{ (((matrix.os == 'linux') && format(' - {0}', (((matrix.manylinux == 'auto') && 'manylinux') \|\| matrix.manylinux))) \|\| '') }})	0s
build pgo-optimized on ${{ matrix.os }} / ${{ matrix.interpreter }}	0s
test build on ${{ matrix.target }}-${{ matrix.distro }}	0s
test build on ${{ matrix.os }}	0s
inspect-pypi-assets	0s
check	2s
release	0s
	48m 6s

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