fix(image-optimizer): respect updated upstream Cache-Control when reusing cached image

[itoudium]

What?

When an optimized image is revalidated and the upstream source bytes are
unchanged (same upstream etag), Next.js reuses the previously optimized buffer
to skip re-encoding. This PR makes that path return the maxAge derived from
the freshly re-fetched upstream Cache-Control header, instead of the
revalidate value persisted in the previous cache entry.

Why?

The optimized-image cache TTL was effectively frozen at the value computed on
the very first fetch. Because the reuse branch returned the previous entry's
revalidate, changing the upstream/CDN Cache-Control max-age was never
reflected as long as the source image bytes stayed identical — each
revalidation kept re-emitting the original TTL.

This reuse optimization was introduced in #67257; the buffer reuse itself is
correct (the optimized output is identical), but carrying over the old TTL was
an unintended side effect.

Reproduction

1. images: { minimumCacheTTL: 0 }, a remote/internal image whose upstream
   responds with Cache-Control: max-age=2.
2. next build && next start, request /_next/image?url=...&w=64&q=75 →
   response Cache-Control reports max-age=2.
3. Let the cache entry go stale (> 2s).
4. Change the upstream to e.g. Cache-Control: max-age=600 without changing
   the image bytes.
5. Request again → the response still reports max-age=2; the new value is
   never picked up.

(Originally observed under output: 'standalone', but this is not specific to
standalone. The bug is in Next.js's built-in Image Optimization cache, which runs
on any self-hosted server (next start and output: 'standalone' alike). It does
not affect managed/minimal-mode deployments, where image optimization is handled
by the platform rather than this code path.)

How?

In imageOptimizer(), the reuse branch returned
opts?.previousCacheEntry?.cacheControl?.revalidate || maxAge. The freshly
computed maxAge (already Math.max(minimumCacheTTL, getMaxAge(upstream.cacheControl)))
is now returned directly, so the buffer reuse / CPU optimization from #67257 is
preserved while the cache TTL tracks the current upstream header.

Added a unit test (test/unit/image-optimizer/image-optimizer-max-age.test.ts)
covering both the updated-max-age case and the minimumCacheTTL clamp. The
test fails before the change (Received: 31536000) and passes after.