fix: chat roles for model responses in chat generators

[Amnah199]

Related Issues

• fixes Check ChatRoles in responses returned by ChatGenerators  #1026

Proposed Changes:

• Updated the role assigned to model responses from system to assistant for both GoogleGeminiChatGenerator and VertexAIGeminiChatGenerator.
• Reviewed and verified consistency across other generators to ensure correct role assignment.

How did you test it?

Ran existing tests

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.

[anakin87]

Looks good, but please also update these other occurrences

wrong usage example:

haystack-core-integrations/integrations/ollama/examples/chat_generator_example.py

Line 14 in 3b05662

ChatMessage.from_system(

wrong tests:

haystack-core-integrations/integrations/amazon_bedrock/tests/test_chat_generator.py

Line 203 in 3b05662

messages = [ChatMessage.from_system("What is the biggest city in United States?")]

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

Line 260 in 3b05662

ChatMessage.from_system(content="It's an arithmetic operation."),

[anakin87]

The changes look good. Feel free to merge.

Having more tests would help.
See #1019 (comment)