fix(nextjs): Return correct lastEventId for SSR pages

[s1gr1d]

Fixes an issue where Sentry.lastEventId() would return undefined after calling captureUnderscoreErrorException() in Next.js error pages. In Next.js SSR, each incoming request gets its own isolation scope (forked automatically).
The function now explicitly sets the event ID on the isolation scope, making it accessible via lastEventId() for displaying error reference IDs to users on custom error pages. Includes a test to verify the fix (test breaks without setting the ID on the isolation scope).

Closes #19217

[github-actions]

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[andreiborza]

As discussed offline: I think this is already happening by the underlying captureException (if it reaches that) so I'm a bit confused why this is needed.

Let's test this in an integration test and see if it's causing an issue there, and maybe we even have to try an ssr e2e test.

[s1gr1d]

Closing this, as this is probably not the core issue. See this comment: #19217 (comment)