fix: allow max_retries to be 0 to disable retries

[stevehodgkiss]

Description

Change max_retries type from :pos_integer to :non_neg_integer so users can set it to 0 to disable retry attempts for transient network errors.

The reason I'd like to disable automatic transient error retries is to have control over how timeouts are handled. Some generations can take 5+ minutes and I'm not sure Req's put_aws_sig_v4 regenerates the signature on timeout retries, so eventually it hits a 403 with a signature error like this:

%ReqLLM.Error.API.Response{reason: "Bedrock API error", response_body: %{"message" => "Signature expired: 20260226T031356Z is now earlier than 20260226T031459Z (20260226T031959Z - 5 min.)"}, status: 403

I'd rather have timeouts not be retried and instead rely on Oban retry limits. Req already supports max_retries: 0, so I think it makes sense to allow that.

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update

Breaking Changes

Testing

• Tests pass (mix test)
• Quality checks pass (mix quality)

Checklist

• My code follows the project's style guidelines
• I have updated the documentation accordingly
• I have added tests that prove my fix/feature works
• All new and existing tests pass
• My commits follow conventional commit format
• I have NOT edited CHANGELOG.md (it is auto-generated by git_ops)

Related Issues

Closes #