[red-knot] Error out when an mdtest code block is unterminated #37384

Job	Run time
cargo build (release)	0s
Determine changes	12s
mkdocs	2m 23s
cargo fmt	21s
python package	1m 11s
pre-commit	1m 5s
cargo clippy	1m 40s
cargo fuzz build	5m 6s
cargo build (msrv)	4m 2s
benchmarks	4m 46s
cargo shear	21s
cargo test (linux)	1m 45s
cargo test (linux, release)	3m 33s
cargo test (wasm)	1m 27s
cargo test (windows)	3m 49s
test scripts	57s
formatter instabilities and black similarity	0s
ecosystem	2m 40s
test ruff-lsp	42s
fuzz parser	0s
	36m 0s

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