Adds CompanyModel, an SDK resource for managing a company's custom LLM registrations.

Until now the SDK had no way to reach them: the only way to use one was to hardcode its technical name (custom-{company_id}-{uuid}) into an ask() call. There was no way to discover what was registered, inspect it, or change it without leaving Python. CompanyModel covers the full lifecycle against /api/v3/company/models: list, get, create, save and delete. It follows the same active-record pattern as ApiKey, so it inherits list/get/refresh/delete and adds create() and save(). Reading is open to any member of the company; creating, updating and deleting require the Company Admin role and raise PermissionDeniedError otherwise.

Three server behaviours are handled explicitly: litellm_model, endpoint and api_key are fixed at creation (the API ignores attempts to change them rather than rejecting them), api_key is write-only and never returned, and the collection endpoint returns a bare array rather than a paginated envelope.