chromium: Fail clearData explicitly instead of faking success

[NAME-ASHWANIYADAV]

Fixes #2061

What

Two coordinated changes:

• The Chromium RuntimeImpl.clearData() stub now returns WResult.fromException(new UnsupportedOperationException(...)) instead of an already-completed success. The TODO stays to signal that real clearing against the Chromium engine is a follow-up.
• SessionStore.clearCache() now uses the two-listener then(valueListener, exceptionListener) overload: on failure it logs the error and still recreates the suspended sessions.

Why

The stub returned WResult.fromValue(null), so callers took their success path with nothing cleared. The user-facing path is Settings → Privacy → "clear cookies and site data" / "clear web content" (PrivacyOptionsView.java:74-83) → SessionStore.clearCache(). On Chromium the sessions suspend and reload, so the action looks like it worked - while cookies, site data and caches are untouched. The Gecko backend really clears (gecko/RuntimeImpl.java:91-92).

Failing the result alone - the way capturePixels was handled in #2025 - would have regressed this path: clearCache() suspends every session before calling clearData(), and recreateSession() lived only inside .then() with no .exceptionally(). An honest failure would have left every session suspended forever. That is why the caller changes in the same PR: with the error branch added, sessions are recreated on both outcomes.

Net behavior:

• Chromium: sessions still suspend and recreate exactly as before; the result now reports the truth and the failure is logged instead of silently faked.
• Gecko: the success path is unchanged; a real clearData failure is now logged and no longer strands suspended sessions (previously an unhandled rejection).

Follow-up (out of scope)

Implementing clearData for real against Chromium's BrowsingDataRemover, mapping WRuntime.ClearFlags onto its data-type mask.

Testing

• Builds: ./gradlew assembleNoapiArm64GeckoGenericDebug
• Existing unit tests pass: ./gradlew testNoapiArm64GeckoGenericDebugUnitTest

[NAME-ASHWANIYADAV]

@svillar PTAL !!

[svillar]

instead of repeating the for loop can we do instead

whenComplete((aVoid, throwable) -> {
            if (throwable != null) {
                Log.e(LOGTAG, "Failed to clear browsing data", throwable);
            }

            for (Session session : activeSession) {
                session.recreateSession();
            }

[NAME-ASHWANIYADAV]

@svillar will appreciate a re-review !!

[NAME-ASHWANIYADAV]

One note: WResult didn't have whenComplete, so I added it as a default method mirroring CompletableFuture.whenComplete (built on the existing two-listener then, so both backends get it for free), and clearCache now uses exactly the shape you suggested. Also rebased on main and added the Fixes line to the commit