fix: send correct field names for sandbox input files

Merge https://github.com/google/adk-python/pull/5958

## Problem

`AgentEngineSandboxCodeExecutor` was building the input-file payload with
incorrect JSON field names:

| Code sent  | API expects |
|------------|-------------|
| `contents` | `content`   |
| `mimeType` | `mime_type` |

This caused all input files to be silently unreadable inside the sandbox,
producing errors such as:

```
pandas.errors.EmptyDataError: No columns to parse from file
```

Fixes #3690

## Changes

- `src/google/adk/code_executors/agent_engine_sandbox_code_executor.py` — rename the two dict keys in the `input_data['files']` list comprehension.
- `tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py` — add regression test `test_execute_code_sends_correct_field_names_for_input_files` that verifies the correct keys are sent to the API.

## Testing plan

- [x] New regression test added that asserts `content` and `mime_type` are used (was failing before the fix, passes after).
- [x] All existing tests in the file still pass.

### pytest output

```
uv run --extra test python -m pytest tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py -v
======================== 11 passed, 5 warnings in 2.04s ========================
```

### pre-commit

```
pre-commit run --files src/google/adk/code_executors/agent_engine_sandbox_code_executor.py \
                       tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py
isort....................................................................Passed
pyink....................................................................Passed
addlicense...............................................................Passed
```

Co-authored-by: Kathy Wu <wukathy@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/5958 from jordanchendev:fix/3690-sandbox-input-file-field-names eefe91165fb5ea156b6e473adb329ce12d13616f
PiperOrigin-RevId: 933868136

src/google/adk/code_executors/agent_engine_sandbox_code_executor.py

@@ -181,7 +181,7 @@ class AgentEngineSandboxCodeExecutor(BaseCodeExecutor):
           {
               'name': f.name,
               'content': f.content,
-              'mimeType': f.mime_type,
+              'mime_type': f.mime_type,
           }
           for f in code_execution_input.input_files
       ]

tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py

@@ -157,7 +157,7 @@ class TestAgentEngineSandboxCodeExecutor:
     )
     sent_files = call_kwargs["input_data"]["files"]
     assert sent_files == [
-        {"name": "data.csv", "content": "a,b,c", "mimeType": "text/csv"}
+        {"name": "data.csv", "content": "a,b,c", "mime_type": "text/csv"}
     ]
 
   @patch("vertexai.Client")
@@ -355,6 +355,54 @@ class TestAgentEngineSandboxCodeExecutor:
         input_data={"code": 'print("hello world")'},
     )
 
+  @patch("vertexai.Client")
+  def test_execute_code_sends_correct_field_names_for_input_files(
+      self,
+      mock_vertexai_client,
+      mock_invocation_context,
+  ):
+    """Input files are sent with 'content' and 'mime_type' keys (not 'contents'/'mimeType')."""
+    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": "", "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="import pandas as pd; df = pd.read_csv('data.csv')",
+        input_files=[
+            File(
+                name="data.csv", content=b"col1,col2\n1,2", mime_type="text/csv"
+            )
+        ],
+    )
+
+    executor.execute_code(mock_invocation_context, code_input)
+
+    mock_api_client.agent_engines.sandboxes.execute_code.assert_called_once_with(
+        name="projects/123/locations/us-central1/reasoningEngines/456/sandboxEnvironments/789",
+        input_data={
+            "code": "import pandas as pd; df = pd.read_csv('data.csv')",
+            "files": [{
+                "name": "data.csv",
+                "content": b"col1,col2\n1,2",
+                "mime_type": "text/csv",
+            }],
+        },
+    )
+
   def test_init_with_agent_engine_resource_name(self):
     """Tests init when only agent_engine_resource_name is provided."""
     agent_engine_name = (