fix(plugin): omit plugin input when copy fails

Return None rather than the shared reference when the execution input
cannot be deep-copied, so handler isolation holds even when a snapshot
cannot be created. Applied from an AI review suggestion.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: wangyb-A

packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/plugin.py

@@ -406,12 +406,11 @@ def _snapshot_execution_input(self, execution_input: Any) -> Any:
         try:
             return copy.deepcopy(execution_input)
         except Exception:
-            # A plugin-facing view must never break the execution. Fall back to
-            # the shared reference rather than dropping the input entirely.
+            # Preserve handler isolation if a snapshot cannot be created.
             logger.exception(
-                "Failed to copy execution input for plugins; passing shared reference"
+                "Failed to copy execution input for plugins; omitting plugin input"
             )
-            return execution_input
+            return None
 
     def on_invocation_end(
         self,