fix: increment `form.pending` count before preflight validation #26954

Job	Run time
lint-all	5m 24s
test-kit-svelte-async (dev)	3m 37s
test-kit-svelte-async (build)	3m 59s
pkg-pr-new	54s
test-kit-server-side-route-resolution (dev)	3m 48s
test-others (24)	2m 18s
test-kit-server-side-route-resolution (build)	4m 8s
test-kit (20, ubuntu-latest, chromium)	12m 19s
test-others (22)	2m 15s
test-others (18)	2m 29s
test-others (20)	2m 39s
test-kit (18, ubuntu-latest, chromium)	12m 36s
test-kit (24, ubuntu-latest, chromium)	10m 51s
test-kit-cross-browser (18, macOS-latest, webkit, dev)	3m 32s
test-kit (22, ubuntu-latest, chromium)	12m 4s
test-kit-cross-browser (18, windows-latest, chromium, dev)	3m 34s
test-kit-cross-browser (18, macOS-latest, webkit, build)	9m 58s
test-kit-cross-browser (18, ubuntu-latest, firefox, dev)	2m 50s
test-kit-cross-browser (18, ubuntu-latest, firefox, build)	6m 3s
test-kit-cross-browser (18, windows-latest, chromium, build)	5m 0s
	1h 50m 18s

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