Add Heabsy as a declarative provider (EU-hosted, OpenAI-compatible)

What problem would this solve? Users who need inference inside the EU/EEA for GDPR or data-residency reasons currently have Scaleway and OVHcloud among the declarative providers. Heabsy is an OpenAI-compatible inference API for open models operated by FEYA, s.r.o. (Slovakia, EU). Models in its EEA tier run on dedicated GPUs in EEA data centres with zero data retention. Adding it as a declarative provider lets goose users select it in goose configure without setting up a custom provider.

What would a good outcome look like? Heabsy appears in the provider list with HEABSY_API_KEY and base URL https://api.heabsy.com/v1, dynamic model listing, and a row in the providers table in the docs.

Possible approaches The same shape as TrustedRouter (#11422): one JSON file in crates/goose-providers/src/declarative/definitions/ with "engine": "openai", one line in the provider list in declarative.rs, and one row in documentation/docs/getting-started/providers.md. No Rust logic changes. I can open the PR once this issue is Ready.

Additional context Model catalog and prices: https://heabsy.com/models. API reference: https://api.heabsy.com/docs. I work on Heabsy.

• I have verified this does not duplicate an existing feature request