`constrained-generators`: soundness tests and bugfixes #13898

Job	Run time
gen-hie	6s
build (8.10.7, ubuntu-latest)	1m 13s
cabal-format	7s
build (9.2.8, ubuntu-latest)	1m 11s
build (9.6.4, ubuntu-latest)	1m 17s
fourmolu	45s
build (9.8.2, ubuntu-latest)	1m 16s
Check branch history	11s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	1s
Send a slack notification on \#ledger-internal if the nightly build failed	1s
Tests completed	1s
	6m 9s

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