fix(core): Inject replayId into trace context for buffer mode errors

In buffer mode, the scope's replayId is empty until captureReplay()
is called. But the event's trace context is built from the
transaction's frozen baggage, which was frozen before captureReplay()
set the replayId. This means error events that trigger a buffer
replay flush are never linked to the replay.

After captureReplay() runs, check if the scope has a replayId that
the trace context is missing and inject it via a new withReplayId
copy method on TraceContext.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: romtsn

sentry/api/sentry.api

@@ -4567,6 +4567,7 @@ public final class io/sentry/TraceContext : io/sentry/JsonSerializable, io/sentr
 	public fun getUserId ()Ljava/lang/String;
 	public fun serialize (Lio/sentry/ObjectWriter;Lio/sentry/ILogger;)V
 	public fun setUnknown (Ljava/util/Map;)V
+	public fun withReplayId (Lio/sentry/protocol/SentryId;)Lio/sentry/TraceContext;
 }
 
 public final class io/sentry/TraceContext$Deserializer : io/sentry/JsonDeserializer {

sentry/src/main/java/io/sentry/SentryClient.java

@@ -254,7 +254,16 @@ private boolean shouldApplyScopeData(final @NotNull CheckIn event, final @NotNul
     }
 
     try {
-      final @Nullable TraceContext traceContext = getTraceContext(scope, hint, event);
+      @Nullable TraceContext traceContext = getTraceContext(scope, hint, event);
+      if (traceContext != null && scope != null) {
+        final @Nullable SentryId replayId = scope.getReplayId();
+        if (replayId != null
+            && !replayId.equals(SentryId.EMPTY_ID)
+            && (traceContext.getReplayId() == null
+                || SentryId.EMPTY_ID.equals(traceContext.getReplayId()))) {
+          traceContext = traceContext.withReplayId(replayId);
+        }
+      }
       final boolean shouldSendAttachments = event != null;
       List<Attachment> attachments = shouldSendAttachments ? getAttachments(hint) : null;
       final @Nullable SentryEnvelope envelope =

sentry/src/main/java/io/sentry/TraceContext.java

@@ -131,6 +131,24 @@ public final class TraceContext implements JsonUnknown, JsonSerializable {
     return replayId;
   }
 
+  @ApiStatus.Internal
+  public @NotNull TraceContext withReplayId(final @NotNull SentryId replayId) {
+    final TraceContext copy =
+        new TraceContext(
+            traceId,
+            publicKey,
+            release,
+            environment,
+            userId,
+            transaction,
+            sampleRate,
+            sampled,
+            replayId,
+            sampleRand);
+    copy.unknown = unknown;
+    return copy;
+  }
+
   // region json
 
   @Nullable