Add models_deepseek() function for listing available DeepSeek models

[jcrodriguez1989]

Summary

Adds the models_deepseek() function to complete the DeepSeek provider implementation, bringing it in line with other providers that have models_*() functions.

Changes

• New function: models_deepseek() that retrieves available models from the DeepSeek API
  • Accepts base_url, api_key, and credentials parameters (consistent with other providers)
• Tests: Added test case "can list models" using the common test_models() helper

Consistency

This implementation follows the established pattern used by other providers:

• Similar signature to models_openai(), models_google_gemini(), etc.
• Integrated into the existing documentation structure

Testing

• ✅ New test added: test_that("can list models", { test_models(models_deepseek) })

[hadley]

Thanks! Can you please add a bullet to the top of NEWS.md? It should briefly describe the change and end with (@yourname, #issuenumber).

[jcrodriguez1989]

Done.
Let me know if further changes are needed.
Related issue: #919