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.