feat: add grok imagine as image generation

[lmariscal]

Summary

• Add image generation support for xAI's Grok Imagine API (grok-imagine-image and grok-imagine-image-pro models)
• Implement ImageGenerationModel trait for the xAI provider, following the same pattern as OpenAI's implementation
• xAI uses aspect_ratio and resolution instead of pixel-based size, so these are passed via additional_params (defaults to "1:1" aspect ratio)

Changes

• rig-core/src/providers/xai/image_generation.rs — New module implementing the ImageGenerationModel trait for xAI
• rig-core/src/providers/xai/mod.rs — Register and re-export the image_generation module (behind image feature flag)
• rig-core/src/providers/xai/client.rs — Change ImageGeneration = Nothing to Capable<ImageGenerationModel<H>>
• rig-core/examples/xai_image_generation.rs — Example demonstrating usage with custom resolution and aspect ratio

Test plan

• cargo clippy --example xai_image_generation --features image passes
• cargo test --lib -p rig-core passes
• Example runs successfully and produces a valid image

Fixes #1517