fix: explicitly set stream=False to prevent SSE issues with some providers

[octo-patch]

Fixes #436

Problem

Some LLM API providers (e.g. kivest/ezif endpoints) default to returning streaming Server-Sent Events (SSE) responses instead of standard JSON. The OpenAI client then receives a raw string that lacks a .choices attribute, causing:

'str' object has no attribute 'choices'

Solution

Add "stream": False explicitly to the non-streaming request params in openevolve/llm/openai.py. This makes the non-streaming behaviour explicit and ensures JSON is always returned regardless of provider defaults. This is standard OpenAI API behaviour and is a no-op for providers that already default to non-streaming.

Testing

• Verified the change applies only to the standard (non-reasoning-model) code path
• The fix is backwards-compatible: providers that already return non-streaming JSON are unaffected

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