fix(next)!: stitchErrorResponse returns Response | undefined so it composes

[rejifald]

What

@stitchapi/next's stitchErrorResponse(err) mapped any thrown value to a Response
(a StitchError → 502, everything else → 500). That made "I handled it" and "not my
error" indistinguishable at the call site: to rethrow a non-stitch error you had to
pre-guard with isStitchError, and the internal 500 branch silently swallowed
unrelated bugs into an HTTP response.

It now returns undefined for a non-StitchError, so map-or-rethrow collapses to one
branch:

} catch (err) {
    const mapped = stitchErrorResponse(err);
    if (mapped) return mapped; // a Stitch failure → safe JSON Response
    throw err;                 // anything else falls through untouched
}

This is the contract @stitchapi/elysia's stitchErrorResponse already ships on main
(stitchErrorResponse(err) ?? new Response(...)); this aligns @stitchapi/next with it.

⚠️ Breaking

stitchErrorResponse(nonStitchError) was a 500 Response; it is now undefined. Pre-GA,
no deprecation. Callers that relied on the catch-all 500 should pass an explicit fallback
(stitchErrorResponse(err) ?? Response.json(…, { status: 500 })).

Changed

• src/index.ts — guard if (!isStitchError(err)) return undefined;, return type
  Response | undefined, drop the now-dead non-stitch 500 branch.
• test/next.spec.ts — flip the non-stitch case to toBeUndefined(); narrow the
  StitchError cases through a small mustRespond helper.
• README.md + integrations/next.mdx — composition pattern; drop the now-redundant
  isStitchError pre-guard from the snippet.

Context

Carved out of the #470 contract-freeze sweep so this contract change lands and is reviewed
on its own. (In #470 it also picks up an ErrorResponseOptions→StitchErrorOptions rename
and a BAD_GATEWAY const — omitted here to keep the diff to the behavior change.)

Verification

pnpm --filter @stitchapi/next test (18 pass) + check:types clean; full pre-push suite
green (incl. build-docs twoslash-compiling the updated next.mdx).

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