[Server] Extract metadata from ExecutionResult in gRPC responses

[thinkclay]

Type: Enhancement

Priority: Medium

Location: crates/radium-core/src/server/radium_service.rs:907

Description:
When ExecutionResult contains metadata (thinking steps, artifacts, structured data), it should be extracted and included in gRPC responses to clients.

Current Behavior:

• ExecutionResult metadata field exists but not extracted
• Clients receive basic success/failure status only
• Thinking steps and artifacts not accessible via gRPC

Expected Behavior:

• Extract metadata when available in ExecutionResult
• Convert to protobuf format for gRPC responses
• Include thinking steps in appropriate response fields
• Expose artifacts to clients

Metadata Types to Extract:

• Thinking steps and reasoning
• Generated artifacts (files, code, etc.)
• Structured data outputs
• Execution traces

Dependencies:

• Protobuf schema may need updates for metadata types
• Proper serialization of complex metadata

Acceptance Criteria:

• Metadata extraction logic implemented
• Protobuf schema updated if needed
• Metadata included in gRPC responses
• Tests verify metadata roundtrip
• Documentation with metadata structure
• Backwards compatibility maintained

Related: Server improvements from TODO_TRACKING.md

Priority: MEDIUM