Feat: LiteLlm: Make LiteLLM file attachment failures consistent by raising explicit errors instead of silent fallback

[Lin-Nikaido]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Feat: LiteLlm: Make LiteLLM file attachment failures consistent by raising explicit errors instead of silent fallback #5546

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Note: I observed unrelated local failures in tests/unittests/cli/utils/test_cli_deploy.py.
The failures seem to be caused by a local environment/import-path difference, not by this PR:

click.exceptions.ClickException: Failed to import agent module
No module named 'test_raises_on_import_error0'

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• I have read the CONTRIBUTING.md document.
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[adk-bot]

Response from ADK Triaging Agent

Hello @Lin-Nikaido, thank you for creating this PR!

Could you please fill out the Testing Plan section in your PR description? This should include details on how you tested your changes, such as unit test results or steps for manual end-to-end testing, as outlined in our contribution guidelines.

This information will help reviewers to review your PR more efficiently. Thanks!

[rohityan]

Hi @Lin-Nikaido , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @GWeale , can you please review this.

[GWeale]

Thanks for the PR. The same behavior already landed in main, where LiteLlm raises an explicit ValueError for unsupported file URIs instead of the silent text fallback (see _is_file_uri_supported and _get_content in src/google/adk/models/lite_llm.py, with matching tests in test_litellm.py), so we are closing this as already resolved.