openai and deepseek providers crash on an empty choices list

Both the OpenAI and DeepSeek providers read response.choices[0].message.content without checking that choices is non-empty:

• keep/providers/openai_provider/openai_provider.py:69
• keep/providers/deepseek_provider/deepseek_provider.py:83

An OpenAI-compatible backend can return HTTP 200 with an empty choices list, for example when a content filter blocks the completion. choices[0] then raises IndexError and the workflow step fails instead of degrading.

This is the same failure fixed for litellm/vllm in #6705 and for grok in #6710; these two providers were not covered.

Reproduction

Call _query with a client whose response has choices == []:

IndexError: list index out of range

Expected

Return an empty string, the way grok/litellm/vllm now do, so an empty completion does not crash the step.