[Server] Integrate OrchestrationService for agent execution

[thinkclay]

Type: Enhancement

Priority: High

Location: crates/radium-core/src/server/radium_service.rs:2021

Description:
The gRPC server's execute_agent endpoint needs to trigger actual agent execution via OrchestrationService. Currently the integration is not fully connected.

Current Behavior:

• execute_agent endpoint exists but lacks orchestration integration
• Agent execution happens through separate code paths
• Events may not be properly routed to clients

Expected Behavior:

• execute_agent triggers OrchestrationService to run agent
• Proper session management and correlation IDs
• Events streamed back to client via EventBridge
• Execution state properly tracked

Dependencies:

• EventBridge infrastructure (completed)
• OrchestrationService API
• Session management system

Acceptance Criteria:

• execute_agent calls OrchestrationService
• Events properly routed to session streams
• Error handling for failed executions
• Tests verify end-to-end execution flow
• Documentation updated with usage examples

Related: Server/orchestration improvements from TODO_TRACKING.md

Priority: HIGH