fix: use Promise<Response> for async kit handler return types

[MathiasWP]

Summary

When svelte-check adds type annotations to SvelteKit route handlers (GET, POST, PUT, PATCH, DELETE, etc.), it uses Response | Promise<Response> as the return type for all functions. However, TypeScript requires async functions to return Promise<T>, not T | Promise<T> (TS1064).

This caused false positive errors in --incremental mode, where tsc runs directly on the generated overlay files. In non-incremental mode these errors were silently filtered out by the TypeScript plugin's inGenerated diagnostic filtering, but in incremental mode tsc reports them directly.

The bug

// Original code
export async function POST({ request }) { ... }

// Generated overlay (before fix) - triggers TS1064
export async function POST({ request }: RequestEvent) : Response | Promise<Response> { ... }

// Generated overlay (after fix) - correct
export async function POST({ request }: RequestEvent) : Promise<Response> { ... }

Changes

• packages/svelte2tsx/src/helpers/sveltekit.ts: In addTypeToFunction, check if the function has an async modifier. If so, extract just the Promise<T> part from the return type union instead of using the full T | Promise<T>.
• packages/svelte2tsx/test/helpers/index.ts: Updated the async GET test to expect Promise<Response> and added a new test for non-async GET to verify it still gets Response | Promise<Response>.
• packages/svelte-check/test-success/: Added an async +server.ts fixture with GET and POST handlers plus the necessary RequestEvent type to the sanity test suite.

Test plan

• Updated unit test upserts GET async function fails without fix, passes with fix
• New unit test upserts GET non-async function passes (non-async still uses Response | Promise<Response>)
• All 366 svelte2tsx tests pass
• All 6 svelte-check sanity tests pass (including incremental cold/warm cache)

[changeset-bot]

🦋 Changeset detected

Latest commit: f003bca

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
svelte-check	Patch
svelte2tsx	Patch
svelte-language-server	Patch
typescript-svelte-plugin	Patch

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[MathiasWP]

We need this fix to be able to use the --incremental flag with svelte-check