Replace misleading IScopeObserver.setBreadcrumbs with clearBreadcrumbs

`IScopeObserver.setBreadcrumbs(Collection)` 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:

```java
public void setBreadcrumbs(@NotNull Collection 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 getsentry/sentry-java#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`

Start with IScopeObserver.java, ScopeObserverAdapter.java, and Scope.java to trace the observer calls, then inspect PersistingScopeObserver.java and the affected tests. Update the API surface and tests named in the issue so clearing uses clearBreadcrumbs(), and verify the batching and scope tests cover the new behavior and no longer express clearing through setBreadcrumbs(emptyList()).

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