Gemini on google-genai: Incorrect handling when responses contain both text and function call parts

[q-nathangrand]

Bug description

Same issue as #4596 but with google-genai 'model'.

Multiple parts can be returned, including some with text and some with function calls.

The code currently drops all function calls if a part without function calls exists.

Expected behavior

Both function calls and text should be accumulated into the AssistantMessage