fix: Remove display_name for non-Vertex file uploads

Merge https://github.com/google/adk-python/pull/1211

### Description

When using the Google.GenAI backend (GEMINI_API), file uploads fail if the `file_data` or `inline_data` parts of the request contain a `display_name`. The Gemini API (non-Vertex) does not support this attribute, causing a `ValueError`.

This commit updates the `_preprocess_request` method in the `Gemini` class to sanitize the request. It now iterates through all content parts and sets `display_name` to `None` if the determined backend is `GEMINI_API`. This ensures compatibility, similar to the existing handling of the `labels` attribute.

Fixes #1182

### Testing Plan

**1. Unit Tests**

- Added a new parameterized test `test_preprocess_request_handles_backend_specific_fields` to `tests/unittests/models/test_google_llm.py`.
- This test verifies:
  - When the backend is `GEMINI_API`, `display_name` in `file_data` and `inline_data` is correctly set to `None`.
  - When the backend is `VERTEX_AI`, `display_name` remains unchanged.
- All unit tests passed successfully.

```shell
pytest ./tests/unittests/models/test_google_llm.py                                    ░▒▓ ✔  adk-python   base   system   21:14:02 
============================================================================================ test session starts ============================================================================================
platform darwin -- Python 3.12.10, pytest-8.3.5, pluggy-1.6.0
rootdir: /Users/leo/PycharmProjects/adk-python
configfile: pyproject.toml
plugins: anyio-4.9.0, langsmith-0.3.42, asyncio-0.26.0, mock-3.14.0, xdist-3.6.1
asyncio: mode=Mode.AUTO, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 20 items

tests/unittests/models/test_google_llm.py ....................                                                                                                                                        [100%]

============================================================================================ 20 passed in 3.19s =============================================================================================
```

**2. Manual End-to-End (E2E) Test**
I manually verified the fix using `adk web`. The test was configured to use a **Google AI Studio API key**, which is the scenario where the bug occurs.

- **Before the fix:**
  When uploading a file, the request failed with the error: `{"error": "display_name parameter is not supported in Gemini API."}`. This confirms the bug.

<img width="968" alt="Screenshot 2025-06-06 at 21 22 35" src="https://github.com/user-attachments/assets/f1ab2db2-d5ec-40fc-a182-9932562b21e1" />

- **After the fix:**
  With the patch applied, the same file upload was processed successfully. The agent correctly analyzed the file and responded without errors.

<img width="973" alt="Screenshot 2025-06-06 at 21 23 24" src="https://github.com/user-attachments/assets/e03228f6-0b7d-4bf9-955a-ac24efb4fb72" />

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/1211 from ystory:fix/display-name d3efebe74aca635a7a255063e64f07cc44016f05
PiperOrigin-RevId: 769278445

src/google/adk/models/google_llm.py

@@ -23,6 +23,7 @@ import sys
 from typing import AsyncGenerator
 from typing import cast
 from typing import TYPE_CHECKING
+from typing import Union
 
 from google.genai import Client
 from google.genai import types
@@ -244,9 +245,18 @@ class Gemini(BaseLlm):
 
   def _preprocess_request(self, llm_request: LlmRequest) -> None:
 
-    if llm_request.config and self._api_backend == GoogleLLMVariant.GEMINI_API:
+    if self._api_backend == GoogleLLMVariant.GEMINI_API:
       # Using API key from Google AI Studio to call model doesn't support labels.
-      llm_request.config.labels = None
+      if llm_request.config:
+        llm_request.config.labels = None
+
+      if llm_request.contents:
+        for content in llm_request.contents:
+          if not content.parts:
+            continue
+          for part in content.parts:
+            _remove_display_name_if_present(part.inline_data)
+            _remove_display_name_if_present(part.file_data)
 
 
 def _build_function_declaration_log(
@@ -324,3 +334,15 @@ Raw response:
 {resp.model_dump_json(exclude_none=True)}
 -----------------------------------------------------------
 """
+
+
+def _remove_display_name_if_present(
+    data_obj: Union[types.Blob, types.FileData, None],
+):
+  """Sets display_name to None for the Gemini API (non-Vertex) backend.
+
+  This backend does not support the display_name parameter for file uploads,
+  so it must be removed to prevent request failures.
+  """
+  if data_obj and data_obj.display_name:
+    data_obj.display_name = None