[Agent] Add Model Router system for intelligent model selection

[wachterjohannes]

Q	A
Bug fix?	yes/no
New feature?	yes/no
Docs?	yes/no
Issues	Fix #...
License	MIT

Implements the Model Router architecture, enabling intelligent model selection based on input characteristics. This allows agents to automatically route requests to appropriate models (e.g., vision models for images, speech models for audio).

Core Infrastructure:

• RouterInterface: Universal contract for all router types
• RoutingResult: Result with model name, optional transformer, reason, and confidence
• TransformerInterface: Contract for input transformation
• RouterContext: Provides platform access, model catalog, and metadata
• SimpleRouter: Callable-based router for flexible routing logic
• ChainRouter: Composite router for trying multiple routers in sequence
• ModelRouterInputProcessor: Integration with Agent's input processing pipeline

Input Enhancement:

• Add optional platform property to Input class (backward compatible)
• Update Agent to pass platform when creating Input
• Platform available to input processors via Input.getPlatform()

Includes 6 examples demonstrating different routing patterns and comprehensive test coverage (14 router tests, all 187 agent tests passing).

Key Features:

• No breaking changes - all changes are backward compatible
• Extensible design - custom routers in <20 lines
• Multi-provider support - works with any Platform implementation
• Transformation support - routers can transform input before invocation
• Default model fallback - routers can access agent's default model
• Capability-based routing - find models supporting specific capabilities

Architecture Decisions:

• Single RouterInterface for all router types enables composition
• Transformer in RoutingResult (router decides both model and transformation)
• Platform passed via Input (no breaking changes to interfaces)
• RouterContext provides metadata including default model
• Start simple with SimpleRouter, add complexity gradually

[wachterjohannes]

@chr-hertel this is my first protoype of the router - don't look to deep into the code - just the architecture stuff and there will be more as soon as we merged the first part.

following things i have planned:

1. Basic routing
2. Rule-based declarative routing (basically modelflow-ai decission tree)
3. AI-powered routing

Another time - i have to refactor this code as soon as we are OK with the patterns behind it :)