fix(runtime-core): ensure slotFlag not deleted when parent has patchFlag

[jh-leong]

close #11241

a minimal reproduction

• This issue was a side effect of the previous fix for edge case Static Slots optimization combined with component instance re-use can lead to stale slot content never being updated. [EDGE CASE] #2893.
• This fix ensures the slots._ flag is only deleted for manually written render functions by checking the parent component's patchFlag.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	89.4 kB (+38 B)	34.2 kB (+13 B)	30.9 kB (+37 B)
vue.global.prod.js	146 kB (+38 B)	53.6 kB (+13 B)	47.8 kB (+115 B)

Usages

Name	Size	Gzip	Brotli
createApp	51.4 kB (+38 B)	20.1 kB (+18 B)	18.3 kB (+15 B)
createSSRApp	54.7 kB (+38 B)	21.5 kB (+11 B)	19.5 kB (+14 B)
defineCustomElement	53.7 kB (+38 B)	20.9 kB (+9 B)	18.9 kB (-15 B)
overall	65.1 kB (+38 B)	25.1 kB (+11 B)	22.7 kB (+10 B)

[edison1105]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

suite	result	latest scheduled
language-tools	❌ failure	❌ failure
nuxt	✅ success	❌ failure
pinia	✅ success	✅ success
primevue	✅ success	✅ success
quasar	✅ success	✅ success
radix-vue	✅ success	✅ success
router	✅ success	❌ failure
test-utils	✅ success	✅ success
vant	✅ success	✅ success
vite-plugin-vue	✅ success	✅ success
vitepress	✅ success	✅ success
vue-i18n	✅ success	✅ success
vue-macros	✅ success	✅ success
vuetify	✅ success	✅ success
vueuse	✅ success	✅ success
vue-simple-compiler	✅ success	✅ success