Fix union validation logic when `extra='allow'` #4724

Job	Run time
test on ubuntu	2m 38s
coverage	2m 2s
test on macos	2m 31s
test on windows	5m 1s
rust benchmarks	3m 24s
build sdist	19s
build-wasm-emscripten	2m 43s
test-debug 3.11	58s
test MSRV	2m 27s
test-debug pypy3.10	1m 31s
lint	59s
test-pydantic-integration	2m 4s
build on ${{ matrix.os }} (${{ matrix.target }} - ${{ matrix.interpreter \|\| 'all' }}${{ (((matrix.os == 'linux') && format(' - {0}', (((matrix.manylinux == 'auto') && 'manylinux') \|\| matrix.manylinux))) \|\| '') }})	0s
test 3.8	2m 37s
test 3.9	2m 56s
build pgo-optimized on ${{ matrix.os }} / ${{ matrix.interpreter }}	0s
test 3.10	2m 43s
test 3.11	2m 30s
test 3.12	3m 7s
test pypy3.9	2m 59s
test pypy3.10	3m 22s
test build on ${{ matrix.target }}-${{ matrix.distro }}	0s
test build on ${{ matrix.os }}	0s
inspect-pypi-assets	0s
check	4s
release	0s
	46m 55s

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