fix: use correct 'content' key in AgentEngineSandboxCodeExecutor input files

The AgentEngineSandboxCodeExecutor builds input file payloads with key 'contents' (plural), but the Vertex AI SDK reads 'content' (singular). This causes file.get('content', b'') to always return empty bytes — all input files are silently created as zero bytes in the sandbox with SUCCESS status returned.

Fixes the same root cause as PR #5505 which fixed this in other files but missed agent_engine_sandbox_code_executor.py.

Ref: #5500, #5505, #5824

Author: Thapza888

src/google/adk/code_executors/agent_engine_sandbox_code_executor.py

@@ -180,7 +180,7 @@ def execute_code(
       input_data['files'] = [
           {
               'name': f.name,
-              'contents': f.content,
+              'content': f.content,
               'mimeType': f.mime_type,
           }
           for f in code_execution_input.input_files