fix: accept Azure assistant file ids

[he-yufeng]

Fixes #5664.

Summary

This updates the LiteLLM file-id path to treat Azure assistant-* ids the same way it already treats file-* ids for OpenAI/Azure providers.

Without this, an Azure-uploaded assistant file id can fall through the file URI fallback path instead of being sent as a Responses API file content block.

Verification

Ran locally on Windows:

python -m py_compile src\google\adk\models\lite_llm.py tests\unittests\models\test_litellm.py
uv run --extra dev pyink --check src\google\adk\models\lite_llm.py tests\unittests\models\test_litellm.py
uv run --no-sync python -m pytest tests\unittests\models\test_litellm.py -q -k "assistant_file_id or file_id_required_preserves_file_id" -o cache_dir=.tmp\pytest-cache
git diff --check

Result:

3 passed, 257 deselected

Note: uv run --extra test ... is currently blocked for me on Windows because the locked yarl==1.24.1 resolution has no Windows wheel. I installed the minimal target-test dependencies in the local venv and ran the focused test path above.

[rohityan]

Hi @he-yufeng , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

[he-yufeng]

Rebased onto current upstream/main and re-ran the focused checks locally.

Validation:

PYTHONUTF8=1 .\.venv\Scripts\python.exe -m pytest tests\unittests\models\test_litellm.py -q --basetemp .tmp\pytest-5759
# 259 passed, 1 skipped

PYTHONUTF8=1 .\.venv\Scripts\python.exe -m pyink src\google\adk\models\lite_llm.py tests\unittests\models\test_litellm.py --check
PYTHONUTF8=1 .\.venv\Scripts\python.exe -m py_compile src\google\adk\models\lite_llm.py tests\unittests\models\test_litellm.py
git diff --check

The previous pre-commit failure looked like stale-main formatting churn; this branch is now a single commit on top of current main.