fix: Replace `unreachable` with `polars_bail` #14325

Job	Run time
check-features	9m 10s
integration-test (ubuntu-latest)	1m 56s
integration-test (windows-latest)	7m 3s
check-wasm	6m 38s
test (ubuntu-latest)	3m 45s
test (windows-latest)	10m 58s
	39m 30s

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