fix(extend-app-start): Read the extended span finish date, not its finished flag

getExtendedEndTime() gated on span.isFinished(), but finishing the extended
span completes the waitForChildren transaction and runs the event processor
re-entrantly within finishExtendedAppStart(), before the span's finished flag
is set. The processor then saw an unfinished span and dropped the app start
measurement whenever the extension finished after the first frame. Read
getFinishDate() (set before the finish callback) instead, which also keeps the
extended end controllable in tests.

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

Author: buenaflor

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

@@ -117,14 +117,17 @@ public void finishTransaction(final @NotNull SentryDate endTimestamp) {
   public @Nullable SentryDate getExtendedEndTime() {
     try (final @NotNull ISentryLifecycleToken ignored = lock.acquire()) {
       final @Nullable ISpan span = extendedSpan;
-      if (span == null || !span.isFinished()) {
+      if (span == null) {
         return null;
       }
       // A deadline timeout would report an artificially inflated duration; suppress the vital
       // instead.
       if (span.getStatus() == SpanStatus.DEADLINE_EXCEEDED) {
         return null;
       }
+      // Read the finish date, not isFinished(): finishing the extended span completes the
+      // waitForChildren transaction and runs the event processor re-entrantly before the span's
+      // finished flag is set, but the finish timestamp is already in place. Null until finished.
       return span.getFinishDate();
     }
   }

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

@@ -188,6 +188,23 @@ class AppStartExtensionTest {
     assertSame(finishDate, ext.extendedEndTime)
   }
 
+  @Test
+  fun `getExtendedEndTime returns the finish date even when the span still reports unfinished`() {
+    // Reproduces the waitForChildren reentrancy: finishing the extended span completes the
+    // transaction and runs the event processor before the span's isFinished() flips, while the
+    // finish timestamp is already set. getExtendedEndTime() must read the finish date, not the
+    // flag.
+    val ext = extension(windowOpen = true)
+    val finishDate = SentryNanotimeDate()
+    val span = mock<ISpan>()
+    whenever(span.isFinished).thenReturn(false)
+    whenever(span.status).thenReturn(SpanStatus.OK)
+    whenever(span.finishDate).thenReturn(finishDate)
+    ext.registerHandOver(span = span)
+    ext.extendAppStart()
+    assertSame(finishDate, ext.extendedEndTime)
+  }
+
   @Test
   fun `clear clears the extension state`() {
     val ext = extension(windowOpen = true)