fix(LiteLlm): recognize assistant- prefix as valid OpenAI file ID

Merge #5758

Azure OpenAI files uploaded with `purpose="assistants"` receive IDs with an `assistant-` prefix (e.g. `assistant-abc123`). `_looks_like_openai_file_id` only checked for the `file-` prefix, causing Azure PDF attachments to be silently dropped from requests instead of being routed through the Responses API content block.

Fix: extend the `startswith` check to include `"assistant-"`.

Also updated `_redact_file_uri_for_log` to return `assistant-<redacted>` for `assistant-` prefixed IDs, consistent with how `file-` IDs are handled.

Fixes #5664

## Testing plan

Added unit tests in `tests/unittests/models/test_litellm.py`:

- `test_looks_like_openai_file_id` (parametrized) covers:
  - `file-abc123` -> True (existing behavior)
  - `assistant-abc123` -> True (new behavior)
  - `https://example.com/file.pdf` -> False
  - `not-a-file-id` -> False
  - empty string -> False
  - `FILE-abc123` -> False (case sensitive)
- `test_redact_file_uri_for_log_openai_prefixes` (parametrized):
  - `file-abc123` -> `file-<redacted>`
  - `assistant-abc123` -> `assistant-<redacted>` (new behavior)
- `test_redact_file_uri_for_log_uses_display_name_when_provided` confirms `display_name` short-circuits redaction.
- `test_redact_file_uri_for_log_http_url_keeps_scheme_and_tail` confirms HTTP URLs still redact host but preserve scheme and filename.

Run locally with:
```
pytest tests/unittests/models/test_litellm.py -k "looks_like_openai_file_id or redact_file_uri_for_log"
```

Co-authored-by: George Weale <gweale@google.com>
COPYBARA_INTEGRATE_REVIEW=#5758 from nileshpatil6:fix/lite-llm-assistant-file-id-prefix 6b2afd7
PiperOrigin-RevId: 933898777

Author: 2 people

src/google/adk/models/lite_llm.py

@@ -395,6 +395,8 @@ def _redact_file_uri_for_log(
   """Returns a privacy-preserving identifier for logs."""
   if display_name:
     return display_name
+  if file_uri.startswith("assistant-"):
+    return "assistant-<redacted>"
   if _looks_like_openai_file_id(file_uri):
     return "file-<redacted>"
   try:

tests/unittests/models/test_litellm.py

@@ -40,11 +40,13 @@
 from google.adk.models.lite_llm import _get_content
 from google.adk.models.lite_llm import _get_provider_from_model
 from google.adk.models.lite_llm import _is_anthropic_model
+from google.adk.models.lite_llm import _looks_like_openai_file_id
 from google.adk.models.lite_llm import _message_to_generate_content_response
 from google.adk.models.lite_llm import _MISSING_TOOL_RESULT_MESSAGE
 from google.adk.models.lite_llm import _model_response_to_chunk
 from google.adk.models.lite_llm import _model_response_to_generate_content_response
 from google.adk.models.lite_llm import _parse_tool_calls_from_text
+from google.adk.models.lite_llm import _redact_file_uri_for_log
 from google.adk.models.lite_llm import _redirect_litellm_loggers_to_stdout
 from google.adk.models.lite_llm import _safe_json_serialize
 from google.adk.models.lite_llm import _schema_to_dict
@@ -5124,3 +5126,44 @@ async def test_generate_content_async_skips_request_log_build_above_debug(
       assert mock_build.called is should_call
   finally:
     litellm_logger.setLevel(original_level)
+
+
+@pytest.mark.parametrize(
+    "file_uri, expected",
+    [
+        ("file-abc123", True),
+        ("assistant-abc123", True),
+        ("https://example.com/file.pdf", False),
+        ("not-a-file-id", False),
+        ("", False),
+        ("FILE-abc123", False),
+    ],
+)
+def test_looks_like_openai_file_id(file_uri, expected):
+  """Both `file-` and `assistant-` (Azure assistants) prefixes count as OpenAI file IDs."""
+  assert _looks_like_openai_file_id(file_uri) is expected
+
+
+@pytest.mark.parametrize(
+    "file_uri, expected",
+    [
+        ("file-abc123", "file-<redacted>"),
+        ("assistant-abc123", "assistant-<redacted>"),
+    ],
+)
+def test_redact_file_uri_for_log_openai_prefixes(file_uri, expected):
+  """OpenAI-style IDs are redacted while preserving the prefix kind."""
+  assert _redact_file_uri_for_log(file_uri) == expected
+
+
+def test_redact_file_uri_for_log_uses_display_name_when_provided():
+  assert (
+      _redact_file_uri_for_log("file-abc123", display_name="my.pdf") == "my.pdf"
+  )
+
+
+def test_redact_file_uri_for_log_http_url_keeps_scheme_and_tail():
+  assert (
+      _redact_file_uri_for_log("https://example.com/path/file.pdf")
+      == "https://<redacted>/file.pdf"
+  )