Add EUrouter as provider

**What problem would this solve?**
I want to set up EUrouter with Goose, but it's not listed as an available provider. I could set it up as a custom provider, but with 170+ models there's too many to list.

**What would a good outcome look like?**
Have EUrouter and their models listed as an available provider.

**Possible approaches**
Share any approaches, constraints, or trade-offs the design discussion should consider.

**Additional context**
Add any other context or screenshots about the feature request here.

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).

Look for provider configuration files in the codebase, likely under a 'providers' directory. Understand how existing providers like OpenAI or Anthropic are integrated, including model listing. Check the project's contribution guidelines and the linked Goose Issues board for readiness. Adding EUrouter will involve defining its API endpoint, authentication, and listing its 170+ models in the provider registry.

Rédigé par le modèle d'indexation à partir du texte de l'issue.