fix(nextjs-config): snapshot sourcemap inputs for Turbopack

[kkumarsatish567-dot]

Problem

Fixes #4667.

Next.js 16.3+ can continue flushing Turbopack's filesystem cache after runAfterProductionCompile starts. @posthog/nextjs-config currently passes the entire .next directory to posthog-cli sourcemap process, so the CLI can observe one set of chunks during injection and a larger set during upload. Chunks created between those scans have no injected chunk ID and can fail the production build with Chunk ID not found.

Changes

• Snapshot the JavaScript files under the compilation output before invoking the PostHog CLI.
• Pass that explicit file list over the CLI's existing stdin mode so injection and upload operate on the same set of chunks.
• Ignore files that materialize after the snapshot for that run instead of letting them invalidate the already-started sourcemap transaction.
• Add regression coverage for a file appearing while CLI processing is in progress, plus .js/.mjs/.cjs, empty-output, and missing-output cases.

This avoids timing sleeps/retries and keeps the fix local to the integration lifecycle race.

Testing

CI requested via this draft PR. The focused unit tests are in packages/nextjs-config/src/utils.spec.ts.

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore