fix(LiteLlm): add fallback user message to prevent empty content error with include_contents='none'

[ishanrajsingh]

This fix addresses an issue in the LiteLlm integration where setting include_contents='none' causes the content payload sent to the LLM provider to be empty, resulting in a BadRequestError.

Changes

• Added fallback user message in generate_content_async when messages list is empty
• Uses same fallback text as BaseLlm._maybe_append_user_content for consistency
• Added comprehensive integration tests with real dict responses for proper serialization testing

Testing

• All 6 tests pass successfully (3 test functions × 2 backends: GOOGLE_AI and VERTEX)
• Tests verify empty content handling with and without tools
• Tests use actual dict responses instead of MagicMock for proper payload testing

Related Issue

Fixes #3242

[gemini-code-assist]

Summary of Changes

Hello @ishanrajsingh, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical issue within the LiteLlm integration where an empty content payload, often triggered by include_contents='none', would lead to a BadRequestError. The solution introduces a robust fallback mechanism that inserts a default user message when no content is provided, ensuring that API calls to LLM providers are always valid. This enhancement significantly improves the stability and error handling of the LiteLlm model, particularly in scenarios where content generation might be implicitly handled or optional.

Highlights

• Bug Fix for Empty Content: Addresses a BadRequestError in LiteLlm when include_contents='none' results in an empty content payload, ensuring robust handling of such scenarios.
• Fallback User Message Implementation: Introduces a fallback user message in generate_content_async for cases where the messages list is empty, maintaining consistency with BaseLlm._maybe_append_user_content.
• Enhanced Integration Testing: Adds comprehensive integration tests that utilize real dictionary responses to thoroughly validate serialization and the correct handling of empty content, both with and without tools.

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[gemini-code-assist]

Code Review

This pull request successfully addresses a BadRequestError in the LiteLlm integration when include_contents='none' by adding a fallback user message. The change is accompanied by a comprehensive new test suite that validates the fix for various scenarios, including with and without tools, which is excellent. My review includes a comment on a potential logic redundancy in the implementation and suggestions to improve conciseness in the new tests.