Fix error when Google returns only empty text parts

[naveen-corpusant]

In a previous commit (#3360), we ignored text parts. On further testing, this actually causes some issues because of the structure of the if/elif/else statements (sorry for not catching this earlier! I'd tested my original revisions, but your code suggestions looked solid and worked in the happy path, but did break when testing again on 1.13 now).

I'd like to add a snapshot test to this PR- the tricky part is that it's hard to get this type of error to repro (happens on one of our tests about 1 in 10 or 15 times). Let me know if you can think of a better way to test this, I'll continue down the line of snapshot testing but wanted to put something up to also notify you that there's an error in 1.13

[DouweM]

@naveen-corpusant Ay, thanks for the heads-up, I was already planning to do another release today so at least the latest version won't be broken for too long. I suppose the issue is that we raise Unsupported response from Gemini in else, so we'd error on empty text 😬

[naveen-corpusant]

nice yeah exactly, the else statement assert breaks things. totally my bad for not testing more extensively on this, I liked your suggestion but should have run it to catch this

[DouweM]

@naveen-corpusant I agree a test would be good; since it's probably impossible to reproduce reliably, I suggest crafting a custom GenerateContentResponse and calling _process_response directly.

[naveen-corpusant]

@DouweM I've addressed your comments, and added two unit tests (one for the stream iterator, and the other for processing messages).

I've also verified that these tests fail on main, and pass on this PR:

failing tests (for reference):
uv run pytest -s tests/models/test_google.py::test_gemini_streamed_response_emits_text_events_for_non_empty_parts

                else:
>                   assert part.function_response is not None, f'Unexpected part: {part}'  # pragma: no cover
E                   AssertionError: Unexpected part: video_metadata=None thought=None inline_data=None file_data=None thought_signature=None function_call=None code_execution_result=None executable_code=None function_response=None text=''

pydantic_ai_slim/pydantic_ai/models/google.py

uv run pytest -s tests/models/test_google.py::test_google_process_response_filters_empty_text_parts

            else:  # pragma: no cover
>               raise UnexpectedModelBehavior(f'Unsupported response from Gemini: {part!r}')
E               pydantic_ai.exceptions.UnexpectedModelBehavior: Unsupported response from Gemini: Part(
E                 text=''
E               )

pydantic_ai_slim/pydantic_ai/models/google.py:842: UnexpectedModelBehavior

[naveen-corpusant]

@DouweM addressed the comments! thanks for the pointer on snapshot(), that's neat :)