fix(types): add undefined to optional properties for exactOptionalProperties type compatibility

[taearls]

Fixes #20948

Fix Plugin Type Compatibility with exactOptionalPropertyTypes

Problem

When TypeScript's exactOptionalPropertyTypes is enabled, users cannot spread Rollup plugins into Vite config:

plugins: [
  {
    enforce: "pre",
    ...virtual({ "foo": "" })  // ❌ Type error
  }
]

Error: Type 'Plugin<any>' is not assignable to type 'PluginOption'

Root Cause

Rollup v4.43.0 added support for exactOptionalPropertyTypes by explicitly marking optional properties with | undefined. Vite's Plugin interface didn't match these updated type definitions, causing incompatibility.

Solution

Add | undefined to Vite's optional hook parameters (ssr, scan) that extend Rollup's plugin hooks. This aligns with Rollup's type definitions while maintaining backward compatibility.

Changes

• resolveId hook: ssr?: boolean → ssr?: boolean | undefined
• load hook: ssr?: boolean → ssr?: boolean | undefined
• transform hook: ssr?: boolean → ssr?: boolean | undefined
• Internal: scan?: boolean → scan?: boolean | undefined

Result

✅ Rollup plugins can be spread into Vite config
✅ Compatible with exactOptionalPropertyTypes: true
✅ Full type compatibility with Rollup v4.43.0+
✅ No breaking changes for existing users

[sapphi-red]

Thank you!