Feature/adk phase1 sdk usability

[YayoiNanoka]

Summary

This PR implements Phase 1 SDK usability improvements for r_agent.

It adds the foundation for loading/configuring agents, dynamically resolving tools, supporting scoped session state, and persisting sessions through a database-backed service.

Changes

• Add AgentLoader, SingleAgentLoader, and MultiAgentLoader
• Migrate agent ownership to Arc<dyn Agent> for shared loader/runtime usage
• Add agent name uniqueness validation
• Add ModelFactory and DefaultModelFactory
• Add GenerateContentConfig
• Add Toolset, StaticToolset, dynamic tool resolution, predicates, and filtering
• Add ToolRegistry and ToolsetRegistry
• Add TomlAgentLoader for loading LlmAgent definitions from TOML config
• Add scoped state support:
  • app:
  • user:
  • temp:
• Discard temp: state from persisted state/event deltas
• Add DatabaseSessionService<S: SessionStore>
• Add SqliteSessionStore
• Add serde serialization for event/content/action data structures

Notes

• Gemini adapter is intentionally not included in this phase.
• Model creation is protocol-based; this phase supports openai-compatible.
• Sandbox runtime is not implemented in this phase.

Verification

• cargo test
• cargo check --examples