Fix flaky PendingTraceBufferTest by removing buffer thread race (#10800)

Fix flaky PendingTraceBufferTest by removing buffer thread race

The "continuation allows adding after root finished" test was flaky
because it started the buffer worker thread before calling
continuation.cancel(). The worker thread could process the enqueued
trace before the continuation was cancelled, causing extra mock
invocations (TooManyInvocationsError).

The fix removes buffer.start() since it's unnecessary: when the
continuation is cancelled and pendingReferenceCount drops to 0 with
rootSpanWritten still false, PendingTrace.write() is called
synchronously on the calling thread. This eliminates the race
condition entirely.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Co-authored-by: devflow.devflow-routing-intake <devflow.devflow-routing-intake@kubernetes.us1.ddbuild.io>

Author: bm1549

dd-trace-core/src/test/groovy/datadog/trace/core/PendingTraceBufferTest.groovy

@@ -268,8 +268,6 @@ class PendingTraceBufferTest extends DDSpecification {
 
   def "continuation allows adding after root finished"() {
     setup:
-    def latch = new CountDownLatch(1)
-
     def trace = factory.create(DDTraceId.ONE)
     def parent = addContinuation(newSpanOf(trace))
     TraceScope.Continuation continuation = continuations[0]
@@ -302,19 +300,20 @@ class PendingTraceBufferTest extends DDSpecification {
     !trace.rootSpanWritten
 
     when:
-    buffer.start()
+    // Don't start the buffer thread here. When the continuation is cancelled,
+    // pendingReferenceCount drops to 0 with rootSpanWritten still false, so
+    // write() is called synchronously on this thread. Starting the buffer
+    // would introduce a race where the worker thread could process the
+    // enqueued trace before continuation.cancel(), causing extra mock
+    // invocations (TooManyInvocationsError).
     continuation.cancel()
-    latch.await()
 
     then:
     trace.size() == 0
     trace.pendingReferenceCount == 0
     trace.rootSpanWritten
-    _ * bufferSpy.longRunningSpansEnabled()
     1 * tracer.writeTimer() >> Monitoring.DISABLED.newTimer("")
-    1 * tracer.write({ it.size() == 2 }) >> {
-      latch.countDown()
-    }
+    1 * tracer.write({ it.size() == 2 })
     _ * tracer.getPartialFlushMinSpans() >> 10
     _ * tracer.getTagInterceptor()
     0 * _