test(codegen): replace integration test assertions with insta snapshots

22 integration tests now use insta::assert_snapshot! instead of
assert!(contains(...)) chains, capturing complete generated Haskell
output for easier review and updating via cargo insta review.

Author: markovejnovic

Cargo.lock

@@ -17,6 +17,7 @@ heck = { workspace = true }
 clap = { version = "4", features = ["derive"] }
 
 [dev-dependencies]
+insta = "1"
 tempfile = "3"
 
 [lints]