Add tests for VertexAIGeminiChatGenerator

[Amnah199]

Is your feature request related to a problem? Please describe.
At the moment, we don't have any tests for VertexAIGeminiChatGenerator, which poses the risk of undetected bugs in future.

Describe the solution you'd like
Similar unit tests as VertexAIGeminiGenerator.

Describe alternatives you've considered
Rely on manual testing.

[anakin87]

I also feel that the GoogleAIGeminiChatGenerator tests can be improved.

An example:

haystack-core-integrations/integrations/google_ai/tests/generators/chat/test_chat_gemini.py

Lines 222 to 225 in 50352b9

	messages += res["replies"] + [ChatMessage.from_function(content=weather, name="get_current_weather")]
	res = gemini_chat.run(messages=messages)
	assert len(res["replies"]) > 0

Here, I think we should check the role and the format of the returned message.