Fix arity detection for arrows returning nested generics #9003

Job	Run time
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	3m 55s
build-compiler (macos-15, 5.3.0, true, darwin-arm64, aarch64-apple-darwin)	4m 58s
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	2m 56s
build-compiler (macos-15-intel, 5.3.0, true, darwin-x64, x86_64-apple-darwin)	8m 43s
build-compiler (windows-2025, 5.3.0, true, win32-x64, x86_64-pc-windows-gnu, .exe)	7m 8s
build-compiler (ubuntu-24.04, ocaml-variants.4.14.2+options,ocaml-option-static, linux-x64, x86_6...	3m 7s
pkg-pr-new	50s
api-docs	1m 28s
test-integration-rewatch (macos-15)	2m 6s
test-integration-rewatch (macos-15-intel)	3m 30s
test-integration-rewatch (windows-2025)	2m 47s
test-integration-rewatch (ubuntu-24.04-arm)	1m 9s
test-integration-rewatch (ubuntu-24.04)	1m 19s
test-installation-npm (macos-15-intel)	27s
test-installation-npm (macos-15)	17s
test-installation-npm (ubuntu-24.04)	13s
test-installation-npm (windows-2025)	50s
test-installation-pnpm (ubuntu-24.04)	14s
test-installation-npm (ubuntu-24.04-arm)	18s
test-installation-pnpm (macos-15-intel)	34s
test-installation-pnpm (macos-15)	19s
test-installation-pnpm (windows-2025)	1m 4s
test-installation-pnpm (ubuntu-24.04-arm)	16s
publish	0s
	48m 28s

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