### What problem does this PR solve?

Add MCP server test endpoint.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Change mcp_client module to rag/utils/conn.

### Type of change

- [x] Refactoring

### What problem does this PR solve?

Allow users to choose which MCP tools are enabled.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Add MCP dashboard functionalities list_tools and test_tool.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Add MCP server dashboard operations.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

This is a cherry-pick from #7781 as requested.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>