Fix syn parsing's continue_parsing_early for assert/proof #6386

Job	Run time
fmt	55s
smoke-test-and-release-windows	9m 1s
smoke-test-and-release-macos-x86	8m 42s
smoke-test-and-release-linux	6m 16s
test-and-release-macos	12m 35s
test-and-release-macos (record-history)	2m 58s
test-and-release-macos (no-std)	3m 7s
test-and-release-macos (no-alloc)	2m 30s
test-and-release-macos (singular)	3m 23s
test-and-release-macos (cvc5)	2m 20s
release	25s
	52m 12s

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