fix: surface lock contention for trailing-mode updateData

Per CodeRabbit review, trailing-mode triggers carrying updateData are
user-visible (latest payload/metadata/tags). Silently dropping them on
lock contention would change run inputs in a way the user can observe.
Re-throw on contention for that subset so the SDK can retry; the
fast-path + quantization changes mean the herd collapses on its own
before we get here.

Co-Authored-By: Eric Allam <eallam@icloud.com>

Author: devin-ai-integration[bot]

internal-packages/run-engine/src/engine/systems/debounceSystem.ts

@@ -656,6 +656,22 @@ return 0
       throw error;
     }
 
+    if (debounce.mode === "trailing" && debounce.updateData) {
+      // Trailing-mode triggers carrying updateData are user-visible: dropping
+      // them silently would mean the eventual run executes against stale
+      // payload/metadata/tags. Surface the lock failure instead so the SDK can
+      // retry and (with the fast-path + quantization in place) the herd
+      // collapses on its own without us hiding data loss.
+      this.$.logger.warn(
+        "handleExistingRun: lock contention with trailing updateData - rethrowing to avoid silently dropping update",
+        {
+          existingRunId,
+          debounceKey: debounce.key,
+        }
+      );
+      throw error;
+    }
+
     this.$.logger.warn(
       "handleExistingRun: lock contention, returning existing run without rescheduling",
       {