This is just an early spike on how we can test uploaded sourcemaps, I'd like to expand this a bit further in follow up PRs for different Next.js scenarios

• Adds a new nextjs-sourcemaps e2e test app that verifies sourcemaps uploads during next build

• The test app builds a minimal Next.js app against a mock Sentry server, then asserts that:

  • Artifact bundles with valid manifests were uploaded
  • JS/sourcemap pairs have matching, valid debug IDs (UUIDs)
  • Sourcemaps contain real mappings and reference app source files
  • Artifact bundle assemble requests target the correct project

• Extracts the mock server and assertion logic into reusable utilities in @sentry-internal/test-utils (startMockSentryServer, assertDebugIdPairs, assertSourcemapMappings, etc.) so other frameworks can reuse them.

  How it works

1. pnpm build starts a mock Sentry server on :3032, then runs next build with @sentry/nextjs configured to upload sourcemaps to it
2. The mock server captures all requests (chunk uploads, artifact bundle assemble, releases) and writes them to disk
3. pnpm test:assert runs assert-build.ts which loads the captured data and runs the assertion suite