Add input validation for tool gating handlers in MCP server

**Problem:**
Tool handlers for `enable_toolset` and `disable_toolset` accept `name` parameter without validation, potentially allowing:
- Empty strings
- Invalid toolset names
- Injection attacks

**Location:**
`src/mcp/mcp-server.js` lines 125 and 139

**Impact:**
Could cause runtime errors or security issues with malformed input.

**Solution:**
Add proper input sanitization and validation before processing toolset names.

**Related PR:** https://github.com/KHAEntertainment/claude-flow/pull/2
**Requested by:** @KHAEntertainment

Read src/mcp/mcp-server.js around lines 125 and 139, then review the related PR for context on the enable_toolset and disable_toolset handlers. Confirm how toolset names are processed and establish validation behavior for empty, invalid, or malicious input; the work is done when malformed names are rejected before processing.

Written by the indexing model from the issue text.