Add observability metrics support for Stability AI image generation

[jee14]

Relates to: #1608

Summary

• Fix deprecated DEFAULT_IMAGE_MODEL (stable-diffusion-v1-6 → stable-diffusion-xl-1024-v1-0)
• Add ObservationRegistry support to StabilityAiImageModel for metrics/tracing
• Add STABILITY_AI to AiProvider enum
• Add ImageModel enum to StabilityAiApi for type-safe model selection
• Add integration test StabilityAiImageModelObservationIT

Changes

Following the same pattern as OpenAiImageModel:

• StabilityAiImageModel: Wrap API call with observation instrumentation
• StabilityAiImageAutoConfiguration: Inject ObservationRegistry and ImageModelObservationConvention
• AiProvider: Add STABILITY_AI("stability_ai") enum value

Test

• Added StabilityAiImageModelObservationIT (requires STABILITYAI_API_KEY env var)