fix: ensure MCP tools are always available in system prompt

- Remove mcpHub dependency check from getUseMcpToolDescription()
- Remove mcpHub dependency check from getAccessMcpResourceDescription()
- MCP tools now appear in system prompt even when MCP hub is unavailable
- Fixes issue where assistant claims no access to MCP tools when servers are installed but not connected
- Connection issues are now handled at execution time, not prompt generation time

Fixes #5936

Author: roomote