Add streaming support for Gemini provider

[niklasmarderx]

What

The Gemini provider in crates/ember-llm/src/gemini.rs may not fully support streaming responses. Add or verify streaming support using Gemini's SSE API.

Why

Streaming makes chat feel responsive — users see tokens as they arrive instead of waiting for the full response.

How

1. Check crates/ember-llm/src/gemini.rs for the current streaming implementation
2. Compare with crates/ember-llm/src/openai.rs which has working streaming
3. Gemini uses streamGenerateContent endpoint with SSE
4. Implement or fix the streaming path
5. Add a test using MockProvider if possible

Reference

• Gemini Streaming API
• crates/ember-llm/src/streaming.rs has the streaming utilities