fix(browser): Skip profiling for unsampled active root spans

[Jxxunnn]

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Closes #25497

With the trace profile lifecycle in @sentry/browser, notifyRootSpanActive() started the profiler for a root span that was already active at setup. It did not check whether that span was sampled. As a result, an unsampled pageload span started UI profiling and sent a profile_chunk, even with tracesSampleRate: 0.

This change returns early in notifyRootSpanActive() when the root span is not recording. This matches the check in the spanStart listener.

Two tests were added:

• A regression test for an unsampled pageload span with the trace profile lifecycle.
• A test for a sampled root span that is already active when the integration is added. Here only notifyRootSpanActive() can start the profiler, so the test covers that path directly. It also checks that the second notification from the timeout does not start another profiler.

Validation (in packages/browser):

• yarn test: 763 tests passed, including the two new tests
• yarn lint: no errors; oxfmt --check passed on the changed files
• The regression test fails without the fix
• The new active-root-span test fails if notifyRootSpanActive() is disabled
• Browser integration tests (Playwright) were not run

[Lms24]

thank you!