feat: add MCP tool server discovery and invocation endpoint

[syn-zhu]

Summary

Add three new MCP tools to the kagent-controller's existing MCP endpoint at :8083/mcp for dynamic tool server discovery and invocation:

Tool	Purpose
list_tool_servers	List all tool servers across RemoteMCPServer, Service (with kagent.dev/mcp-service=true), and MCPServer CRs
list_tools	Connect to a specific tool server and return its available tools
call_tool	Invoke a specific tool on a specific tool server

Motivation

This was originally proposed as kagent-dev/kmcp#123 but the reviewer correctly noted that it belongs in kagent instead of kmcp, because:

1. kagent already watches all three tool source types — RemoteMCPServer, Service, and MCPServer (optional)
2. kagent already has an MCP handler at :8083/mcp with list_agents / invoke_agent
3. kmcp should stay focused on deployment — translating MCPServer CRs into Kubernetes resources

Moving to kagent provides unified discovery across all tool sources, not just MCPServer CRs.

Design

• Ref format: Kind/namespace/name (e.g. RemoteMCPServer/default/my-server, Service/tools/prometheus, MCPServer/default/weather)
• Session caching: sync.Map with evict-and-retry on stale connections
• URL derivation: Reuses existing ConvertServiceToRemoteMCPServer and ConvertMCPServerToRemoteMCPServer from the translator package
• MCPServer CRD is optional: Gracefully returns empty list if kmcp is not installed