fix(extend-app-start): Consume stored app start trace on the extension path

The first activity continuing an extended app.start did not clear the stored
trace headers (only the headless-follow path did), so a later activity saw
them and wrongly reused the finished app start trace. Clear them on the
extension path too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: buenaflor

sentry-android-core/src/main/java/io/sentry/android/core/ActivityLifecycleIntegration.java

@@ -351,8 +351,8 @@ private void startTracing(final @NotNull Activity activity) {
                   transactionOptions);
         }
 
-        if (isFollowingHeadlessAppStart) {
-          // Consume the stored headless app-start trace so it isn't reused by another activity.
+        if (isFollowingHeadlessAppStart || extensionActive) {
+          // Consume the stored app-start trace so a later activity doesn't reuse it.
           AppStartMetrics.getInstance().setAppStartTraceId(null);
           AppStartMetrics.getInstance().setAppStartSentryTraceHeader(null);
           AppStartMetrics.getInstance().setAppStartBaggageHeader(null);

sentry-android-core/src/test/java/io/sentry/android/core/ActivityLifecycleIntegrationTest.kt

@@ -384,6 +384,37 @@ class ActivityLifecycleIntegrationTest {
     )
   }
 
+  @Test
+  fun `extended app start trace is not reused by a later activity`() {
+    val sut =
+      fixture.getSut {
+        it.tracesSampleRate = 1.0
+        it.isEnableStandaloneAppStartTracing = true
+      }
+    sut.register(fixture.scopes, fixture.options)
+
+    setAppStartTime()
+    AppStartMetrics.getInstance().appStartExtension.extendAppStart()
+
+    val firstActivity = mock<Activity>()
+    sut.onActivityCreated(firstActivity, fixture.bundle)
+    val appStartTraceId =
+      fixture.createdTransactions
+        .single { it.spanContext.operation == ActivityLifecycleIntegration.STANDALONE_APP_START_OP }
+        .spanContext
+        .traceId
+
+    AppStartMetrics.getInstance().appStartExtension.finishExtendedAppStart()
+    AppStartMetrics.getInstance().onAppStartSpansSent()
+
+    val secondActivity = mock<Activity>()
+    sut.onActivityPaused(firstActivity)
+    sut.onActivityCreated(secondActivity, fixture.bundle)
+
+    // The second activity must not continue the already-finished extended app.start trace.
+    assertNotEquals(appStartTraceId, fixture.createdTransactions.last().spanContext.traceId)
+  }
+
   @Test
   fun `extended standalone app start transaction stays open until finishExtendedAppStart`() {
     val sut =