Replace misleading IScopeObserver.setBreadcrumbs with clearBreadcrumbs

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IScopeObserver.setBreadcrumbs(Collection<Breadcrumb>) reads like a bulk setter — "replace the observed breadcrumbs with this collection" — but that isn't its contract. Its real meaning is "the breadcrumbs were cleared", encoded as "the collection I passed you happens to be empty."

PersistingScopeObserver is the only implementation that does anything, and it never looks at the elements:

public void setBreadcrumbs(@NotNull Collection<Breadcrumb> breadcrumbs) {
  if (breadcrumbs.isEmpty()) {
    // ...enqueue a clear...
  }
  // non-empty: silently do nothing
}

Why this is a problem

1. The parameter is a boolean in disguise. The only thing read off the collection is isEmpty(). A non-empty argument is silently ignored, so anyone who takes the name at face value and writes a bulk-replace implements something the SDK never drives that way.
2. It hides a real race. Scope.clearBreadcrumbs() clears the live queue and then hands that same live queue to the observers. If another thread adds a breadcrumb between the clear() and the observer loop, the collection is no longer empty, PersistingScopeObserver does nothing, and the pre-clear breadcrumbs survive on disk — cleared in memory, not cleared in the scope cache. The signal is carried by mutable state observed after the fact instead of by the call itself. Narrow in practice (clearBreadcrumbs() isn't on a hot path), and the symptom is stale breadcrumbs on the next ANR/crash report rather than anything immediately user-visible.
3. It's called constantly for nothing. Scope.addBreadcrumb invokes both addBreadcrumb and setBreadcrumbs on every observer for every breadcrumb. The second call can only ever be a no-op (the collection just had an element added), so we pay an extra virtual call per observer per breadcrumb on a hot path — and it's why the batching work in Batch or defer scope-persistence disk writes during startup #5714 needed extra care around clear ordering.
4. It's inconsistent with the rest of the interface. Attachments already do this correctly: addAttachment / clearAttachments. Breadcrumbs are the odd one out.
5. The name collides with a genuine setter. SentryBaseEvent.setBreadcrumbs(List) really does replace the list, so the same name means two different things depending on the receiver.

Proposed change

• Add clearBreadcrumbs() to IScopeObserver and ScopeObserverAdapter.
• Scope.clearBreadcrumbs() calls observer.clearBreadcrumbs(); Scope.addBreadcrumb drops its observer.setBreadcrumbs(...) call entirely.
• PersistingScopeObserver.setBreadcrumbs becomes clearBreadcrumbs(), unconditionally enqueueing the clear marker — which also removes the race in (2).
• Remove setBreadcrumbs from IScopeObserver. It's public API and not @ApiStatus.Internal, hence filing this against the 9.0 major.
• Fix IScopeObserver's javadoc, which claims all methods are default — none of them are.
• Update ScopeTest, PersistingScopeObserverTest, and PersistingScopeObserverBatchingTest, which currently express "clear" as setBreadcrumbs(emptyList()).

Notes

NdkScopeObserver implements addBreadcrumb but not setBreadcrumbs, so native breadcrumbs are never cleared today. A clearBreadcrumbs() on the interface makes that gap visible; there's no corresponding entry point on INativeScope, so closing it would be follow-up work.

Affected files: sentry/src/main/java/io/sentry/IScopeObserver.java, ScopeObserverAdapter.java, Scope.java, cache/PersistingScopeObserver.java, sentry/api/sentry.api

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JAVA-664