JSX PPX: add Jsx.element return constraint #8684

Job	Run time
build-compiler (macos-15-intel, 5.3.0, true, darwin-x64, x86_64-apple-darwin)	5m 55s
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	3m 10s
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	2m 0s
build-compiler (windows-2022, 5.3.0, true, win32-x64, x86_64-pc-windows-gnu)	4m 50s
build-compiler (macos-15, 5.3.0, true, darwin-arm64, aarch64-apple-darwin)	3m 1s
build-compiler (ubuntu-24.04, ocaml-variants.4.14.2+options,ocaml-option-static, linux-x64, x86_6...	3m 10s
test-installation-pnpm	0s
api-docs	0s
test-installation-npm	0s
publish	0s
pkg-pr-new	0s
test-integration-rewatch	0s
	22m 6s

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