JSX PPX: add Jsx.element return constraint #8709

Job	Run time
build-compiler (ubuntu-24.04, ocaml-variants.4.14.2+options,ocaml-option-static, linux-x64, x86_6...	15m 26s
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	5m 4s
build-compiler (macos-15-intel, 5.3.0, true, darwin-x64, x86_64-apple-darwin)	9m 11s
build-compiler (windows-2022, 5.3.0, true, win32-x64, x86_64-pc-windows-gnu)	5m 25s
build-compiler (macos-15, 5.3.0, true, darwin-arm64, aarch64-apple-darwin)	4m 31s
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	13m 47s
api-docs	2m 23s
pkg-pr-new	1m 5s
test-integration-rewatch (ubuntu-24.04)	1m 17s
test-integration-rewatch (macos-15-intel)	3m 53s
test-installation-pnpm (ubuntu-24.04)	20s
test-installation-pnpm (windows-2022)	42s
test-integration-rewatch (macos-15)	1m 49s
test-installation-pnpm (macos-15-intel)	44s
test-integration-rewatch (ubuntu-24.04-arm)	1m 16s
test-integration-rewatch (windows-2022)	2m 42s
test-installation-pnpm (ubuntu-24.04-arm)	18s
test-installation-npm (macos-15)	20s
test-installation-npm (macos-15-intel)	28s
test-installation-npm (windows-2022)	1m 1s
test-installation-npm (ubuntu-24.04-arm)	18s
test-installation-npm (ubuntu-24.04)	21s
test-installation-pnpm (macos-15)	16s
publish	-1s
	1h 12m 36s

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