refactor(llm-router): consolidate into SemanticRouter with backward compatibility

Per PR review feedback, refactored LLM Router to integrate into SemanticRouter
instead of being a separate class. This maintains the powerful LLM routing
features while keeping the codebase cleaner and more maintainable.

Key changes:
- Extended Route/RouteMatch schemas with optional model, confidence, alternatives
- Added cost optimization support to RoutingConfig
- Implemented from_pretrained() for loading routers with pre-computed embeddings
- Created AsyncSemanticRouter with full async support
- Added backward compatibility wrappers (LLMRouter → SemanticRouter)
- Migrated pretrained configs to router/pretrained/

Backward compatibility:
- Old imports (from redisvl.extensions.llm_router) still work with deprecation warnings
- Parameter mapping (tiers → routes) handled transparently
- Serialization maintains "tiers" format for compatibility
- All existing tests pass (54/54)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: bsbodden