fix(language-core): make generic component internal context inference type-safe across .d.ts boundary

[Holiden]

What

In generateGeneric (packages/language-core/lib/codegen/script/scriptSetup.ts), the emitted return type of a generic component uses __ctx?: Awaited<typeof __VLS_setup> without a NonNullable wrapper, while the props, ctx and exposed parameter positions already wrap it as NonNullable<Awaited<typeof __VLS_setup>>. This wraps __ctx to match.

- { __ctx?: Awaited<typeof __VLS_setup> }
+ { __ctx?: NonNullable<Awaited<typeof __VLS_setup>> }

Why

Event-handler parameters on a generic component are typed as implicit any (TS7006 under noImplicitAny) in a consumer when the component is consumed across the emitted .d.ts boundary (a built package / Nuxt layer), even though slot props are inferred correctly.

In the live virtual code __VLS_setup is a parameter with a default value, so typeof __VLS_setup has no undefined. On declaration emit, TypeScript turns the defaulted parameter into an optional one (__VLS_setup?: Promise<…>), so typeof __VLS_setup now includes undefined, and Awaited<Promise<Setup> | undefined> becomes Setup | undefined. The consumer helper __VLS_FunctionalComponentProps extracts props via a single-step nested inference K extends { __ctx?: { props?: infer P } }, which cannot infer P through the inner | undefined, so props resolves to never. With never props, __VLS_NormalizeComponentEvent falls back and the event handler loses its contextual type → implicit any.

Slot props are unaffected because they go through __VLS_FunctionalComponentCtx, which infers the whole __ctx and applies NonNullable to it. This change makes the return-type __ctx consistent with the parameter positions and with that helper.

This completes #4577, which added the same NonNullable wrapping to the parameter positions but left the return-type __ctx.

Reproduction

Reproduces only across the emitted .d.ts boundary (a same-project live-source test does not trigger it):

1. A <script setup generic="T"> component with defineEmits<{ change: [e: SomeType] }>().
2. Emit its declaration (vue-tsc --declaration --emitDeclarationOnly --noCheck).
3. A consumer that imports the emitted .d.ts and writes @change="(e) => …".
4. vue-tsc --noEmit under noImplicitAny reports TS7006 on e without this change, and is clean with it.

No runtime impact: in the live (non-emitted) path the wrapper is a no-op because typeof __VLS_setup has no undefined.