Abstract and implement Safety Settings across providers

[luckenco]

The current Gemini provider implementation (src/provider/gemini.rs) lacks support for safetySettings.

Instead of just adding it for Gemini, we should create a cross-provider abstraction in the core StructuredRequest or GenerationConfig.

Requirements:

1. Define a provider-agnostic safety configuration in src/core/types.rs (or similar).
   • Needs generic categories (e.g., Hate Speech, Dangerous Content).
   • Needs generic thresholds (e.g., Block None, Block Low, Block Medium).
2. Map these generic settings to provider-specific formats.
   • Implement mapping for Gemini (SafetySetting with category and threshold).
   • (Optional) Investigate/Stub mappings for other providers.
3. Update GeminiRequest to include the safetySettings field.