fix(compiler-core): create FOR node in AST when v-for + v-slot are combined

[Copilot]

When <template v-for v-slot> is used, the AST incorrectly keeps both directives on an ELEMENT node instead of creating a FOR node wrapping an ELEMENT with slot directive. This breaks AST consumers like Volar.

<Foo>
  <template v-for="i in 3" v-slot />
</Foo>

Before: ELEMENT(directives: for, slot)
After: FOR → ELEMENT(directives: slot)

Changes

• transform.ts: Allow v-for structural transform to proceed when v-slot is present (previously skipped all structural directives)
• vFor.ts:
  • Skip codegen for <template v-for v-slot> (handled by buildSlots)
  • Preserve template element as FOR child when it has v-slot
• vSlot.ts: Handle FOR nodes containing templates with v-slot in buildSlots

Test

Added vForSlotIssue.spec.ts verifying correct AST structure.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>compiler-dom create unexpected nodes with v-for + v-slot</issue_title>
<issue_description>### Version
3.0.9

Reproduction link

https://vue-next-template-explorer.netlify.app/#%7B%22src%22%3A%22%3CFoo%3E%5Cn%20%20%3Ctemplate%20v-for%3D%5C%22i%20in%203%5C%22%20%2F%3E%5Cn%20%20%3Ctemplate%20v-for%3D%5C%22i%20in%203%5C%22%20v-slot%20%2F%3E%5Cn%3C%2FFoo%3E%22%2C%22options%22%3A%7B%22mode%22%3A%22module%22%2C%22filename%22%3A%22Foo.vue%22%2C%22prefixIdentifiers%22%3Afalse%2C%22optimizeImports%22%3Afalse%2C%22hoistStatic%22%3Afalse%2C%22cacheHandlers%22%3Afalse%2C%22scopeId%22%3Anull%2C%22inline%22%3Afalse%2C%22ssrCssVars%22%3A%22%7B%20color%20%7D%22%2C%22bindingMetadata%22%3A%7B%22TestComponent%22%3A%22setup-const%22%2C%22setupRef%22%3A%22setup-ref%22%2C%22setupConst%22%3A%22setup-const%22%2C%22setupLet%22%3A%22setup-let%22%2C%22setupMaybeRef%22%3A%22setup-maybe-ref%22%2C%22setupProp%22%3A%22props%22%2C%22vMySetupDir%22%3A%22setup-const%22%7D%7D%7D

Steps to reproduce

Check the console for the AST

What is expected?

AST tree is:

• ROOT
  • ELEMENT
    • FOR
    • FOR
      • ELEMENT(directives: slot)

What is actually happening?

AST tree is:

• ROOT
  • ELEMENT
    • FOR
    • ELEMENT(directives: for, slot)

---

Related: vuejs/language-tools#110

Comments on the Issue (you are @copilot in this section)

@johnsoncodehk Can we change this implement if this is not the design?

• Fixes compiler-dom create unexpected nodes with v-for + v-slot vuejs/core#3498

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