feat(cohere): add timeout and max_retries to chat generator

[Keyur-S-Patel]

Related Issues

• partially fixes #9309

Proposed Changes:

• Added timeout and max_retries to CohereChatGenerator.__init__ as keyword-only params (aligned with Anthropic pattern).
• Implemented retry behavior using provider SDK-compatible httpx transport injection:
  • sync: HTTPTransport(retries=max_retries)
  • async: AsyncHTTPTransport(retries=max_retries)
• Passed custom httpx_client into ClientV2 / AsyncClientV2 when max_retries is set.
• Kept backward compatibility for extra generation kwargs (**kwargs merged into generation_kwargs).
• Updated to_dict/from_dict expectations for timeout and max_retries.
• Updated CohereGenerator tests to reflect current inherited behavior (generation_kwargs instead of removed model_parameters).

How did you test it?

• hatch run test:unit tests/test_chat_generator.py
• hatch run test:unit tests/test_generator.py
• hatch run test:unit tests (full Cohere unit suite)

All unit tests passed on this branch.

Notes for the reviewer

• max_retries is implemented via httpx transport retries because direct max_retries constructor support on ClientV2 is not consistent across environments/versions.
• This PR also includes test alignment in tests/test_generator.py for current CohereGenerator behavior.

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:.

[Keyur-S-Patel]

@anakin87 Can you help me close this PR, its similar to the one you reviewed here

[Keyur-S-Patel]

Gentle reminder about pr review @julian-risch