chore!: CohereChatGenerator - remove **kwargs init parameter + fix test

[anakin87]

Related Issues

• failing test - https://github.com/deepset-ai/haystack-core-integrations/actions/runs/22930602649/job/66551043244: in this test, we pass to the Cohere client a stream parameter that they recently dropped
• I realized that CohereChatGenerator exposes **kwargs in __init__, which are then forwarded to Cohere client; this is a duplicate of generation_kwargs.

Proposed Changes:

• fix failing test by removing unnecessary stream parameter
• remove the confusing **kwargs parameter (breaking change)
• make sure that Generator (which inherits from Chat Generator) still works correctly
• other minor improvements
  • rely on default secrets serde
  • use a more reliable (and cheap) model for test_pipeline_with_cohere_chat_generator that was frequently re-run

How did you test it?

CI

Notes for the reviewer

I'd release a new major version because of the breaking change

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:.

[davidsbatista]

LGTM!

nit: maybe is worth it to also use cheaper models for other live tests? test_live_run_with_mixed_tools

[anakin87]

nit: maybe is worth it to also use cheaper models for other live tests? test_live_run_with_mixed_tools

Done in 57074b8

This also seems to have reduced test time significantly: on Linux/Python 3.13, from 2m30s to 54s