[nextjs] Extend default sourcemaps ignore (#18938) to client-side stub chunks that never emit sourcemaps

[sakuga-cmd]

Problem Statement

@sentry/nextjs 10.43.0 with a Next.js 15.5.14 (webpack) App Router project: the client-side source map upload prints dozens of warnings for chunks that by construction never emit sourcemaps:

~/b0f41f6b-e36f-409b-817f-921f7d295a7b-155.js (no sourcemap found, debug id b0f41f6b-...)
  - warning: could not determine a source map reference (Could not auto-detect referenced sourcemap for ~/b0f41f6b-...-155.js)

In our production builds these were ~95 warnings per deploy. We traced every one of them to two categories of client chunks:

1. App Router client stubs — static/chunks/app/**/route-<hash>.js (one per API route / metadata route), loading-<hash>.js, _not-found/page-<hash>.js, and page-<hash>.js stubs of fully server-rendered pages. Each is ~600–980 bytes: the Sentry debug-id snippet plus a single empty webpack module ({84441:()=>{}}). Webpack emits no .map for them.
2. Tiny lazy plugin chunks — numbered chunks of ~600 bytes (in our case Capacitor web plugin stubs like SplashScreenWeb, HapticsWeb registered via registerPlugin(..., { web: () => import(...) })). Also emitted without .map.

PR #18938 (released in 10.37.0, closing #18872) already solved exactly this class of noise for framework internals/manifests, with the rationale “these will never emit sourcemaps and we just waste time processing these files”. The client-side stubs above are the same situation, but they are not covered by the default ignore list.

User-side workaround status: sourcemaps.ignore globs work for the named stubs (**/static/chunks/app/**/route-*.js, loading-*.js, _not-found/** — this removed ~89 of ~95 warnings for us, merging with the defaults as expected). However the numbered lazy chunks cannot be targeted by any stable glob — their names are content-hashed numerics that change between builds, so a handful of warnings per deploy remain unfixable from userland.

Solution Brainstorm

• Extend the default ignore list approach of feat(nextjs): Update default ignore list for sourcemaps #18938 to App Router client stubs (static/chunks/app/**/route-*.js, loading-*.js, _not-found/**), and/or
• More generally: skip the “could not determine a source map reference” warning for chunks for which webpack emitted no sourcemap and that contain no module code beyond the injected debug-id snippet — those can never benefit from an artifact upload anyway.

Environment: @sentry/nextjs 10.43.0, @sentry/webpack-plugin 5.2.0, Next.js 15.5.14 (webpack build, not Turbopack), widenClientFileUpload: true, deployed on Vercel.

[linear-code]

JS-3006