feat: Add schema casting to prediction responses

[kaushikb11]

PR Description

Add automatic schema casting functionality to prediction responses across all prediction types
(image, document, video, audio). This allows responses to be automatically cast to either:

• Domain-specific Pydantic models from the hub registry
• Custom schemas provided via GenerationConfig

Key changes:

• Add SchemaCastMixin for shared schema casting logic
• Add vlmrun-hub as a core dependency