next.js

7395b29c - Fix isStableBuild() treating commit preview tarballs as stable (#98811)

Fix isStableBuild() treating commit preview tarballs as stable (#98811) ## Summary Commit preview tarballs carry versions like `16.4.0-preview-84cee7e6-20260917` (no `canary` substring), so `isStableBuild()` treated them as stable and canary-only defaults (`turbopackSharedRuntime`, `turbopackMangleExportNames`) were disabled on what are canary-quality builds. Versions containing `-preview-` now count as unstable; numbered npm previews like `16.3.0-preview.10` stay stable. ## Verification - Added `canary-only-config-error.test.ts` covering the version matrix (fails on the tarball case without the fix) - `npx jest packages/next/src/server/config.test.ts` (existing consumer) passes <!-- NEXT_JS_LLM --> <!-- fleet ae573e35-b551-453d-af1c-548ffb598944 --> Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>