fix(nextjs): skip custom browser tracing setup for bot user agents (#20263)

Exposes `isBotUserAgent()` from the browser tracing integration to use
it in the Next.js browser tracing wrapper so bot user agents also skip
Next.js-specific router instrumentation which adds its own interval
timer.

This is intentionally the smallest change to test whether this fixes the
Googlebot rendering issue.

Longer term, we probably want a more general mechanism for integrations
to skip their own setup, with a skip decision that can be downstreamed
easily through wrappers and integration variants instead of checking the
bot predicate ad hoc in each package.

closes #19670

Co-authored-by: GPT-5.4 <noreply@anthropic.com>