test: add regression test for input_files content key (#5912)

Verifies that AgentEngineSandboxCodeExecutor sends input_files with the 'content' key (singular), matching what the Vertex AI Sandbox API expects. Without this fix, file.get('content', b'') returns empty bytes and all input files are silently created empty.

Ref: #5500, #5505, #5824

Author: Thapza888

tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py

@@ -20,6 +20,7 @@
 from google.adk.agents.invocation_context import InvocationContext
 from google.adk.code_executors.agent_engine_sandbox_code_executor import AgentEngineSandboxCodeExecutor
 from google.adk.code_executors.code_execution_utils import CodeExecutionInput
+from google.adk.code_executors.code_execution_utils import File as InputFile
 from google.adk.sessions.session import Session
 import pytest
 
@@ -125,6 +126,63 @@ def test_execute_code_success(
         input_data={"code": 'print("hello world")'},
     )
 
+  @patch("vertexai.Client")
+  def test_execute_code_input_files_content_key(
+      self,
+      mock_vertexai_client,
+      mock_invocation_context,
+  ):
+    """Tests that input_files are sent with 'content' (singular), not 'contents'.
+
+    Regression test for https://github.com/google/adk-python/issues/5500.
+    The Vertex AI Sandbox API reads file.get("content", b""), so using the
+    key "contents" silently creates empty files.
+    """
+    mock_api_client = MagicMock()
+    mock_vertexai_client.return_value = mock_api_client
+    mock_response = MagicMock()
+    mock_json_output = MagicMock()
+    mock_json_output.mime_type = "application/json"
+    mock_json_output.data = json.dumps(
+        {"msg_out": "ok", "msg_err": ""}
+    ).encode("utf-8")
+    mock_json_output.metadata = None
+    mock_response.outputs = [mock_json_output]
+    mock_api_client.agent_engines.sandboxes.execute_code.return_value = (
+        mock_response
+    )
+
+    executor = AgentEngineSandboxCodeExecutor(
+        sandbox_resource_name="projects/123/locations/us-central1/reasoningEngines/456/sandboxEnvironments/789"
+    )
+    code_input = CodeExecutionInput(
+        code='print("hello world")',
+        input_files=[
+            InputFile(
+                name="test.txt",
+                content=b"hello world",
+                mime_type="text/plain",
+            )
+        ],
+    )
+    executor.execute_code(mock_invocation_context, code_input)
+
+    # Verify the API was called with 'content' key, NOT 'contents'
+    expected_input_data = {
+        "code": 'print("hello world")',
+        "files": [
+            {
+                "name": "test.txt",
+                "content": b"hello world",
+                "mimeType": "text/plain",
+            }
+        ],
+    }
+    mock_api_client.agent_engines.sandboxes.execute_code.assert_called_once_with(
+        name="projects/123/locations/us-central1/reasoningEngines/456/sandboxEnvironments/789",
+        input_data=expected_input_data,
+    )
+
   @patch("vertexai.Client")
   def test_execute_code_recreates_sandbox_when_get_returns_none(
       self,