<fix>[thread]: guard Context.current() with telemetry check

SyncTaskFuture constructor calls Context.current() unconditionally,
triggering ServiceLoader for ContextStorageProvider even when
telemetry is disabled. If sentry-opentelemetry-bootstrap jar is
on classpath, ServiceLoader fails with "not a subtype" due to
ClassLoader isolation in Tomcat, throwing ServiceConfigurationError
(extends Error) that escapes all catch(Exception) blocks.

1. Why is this change necessary?
MN startup crashes with ORG_ZSTACK_CORE_WORKFLOW_10001 because
Context.current() triggers ServiceLoader unconditionally in
SyncTaskFuture constructor, even when telemetry is disabled.

2. How does it address the problem?
Only call Context.current() when isTelemetryEnabled() returns
true, matching the existing guard pattern used in other
DispatchQueueImpl code paths (lines 351, 1069).

3. Are there any side effects?
None. When telemetry is disabled, parentContext was never used.

# Summary of changes (by module):
- core/thread: conditionalize Context.current() in SyncTaskFuture

Related: ZSTAC-82274
Change-Id: I8ae1f790c8591ae7acebd011319878eb824d2fb0

Author: PandaWuu

core/src/main/java/org/zstack/core/thread/DispatchQueueImpl.java

@@ -302,7 +302,7 @@ private class SyncTaskFuture<T> extends AbstractFuture<T> {
 
         public SyncTaskFuture(SyncTask<T> task) {
             super(task);
-            this.parentContext = Context.current();
+            this.parentContext = isTelemetryEnabled() ? Context.current() : null;
         }
 
         private SyncTask getTask() {