Make AI features (AI Copilot + MCP) optional via [ai-features] extra

- Move fastmcp from core requirements.txt to mcp-requirements.txt
- Add MCP_AVAILABLE feature flag in agent/__init__.py (graceful degradation)
- Guard MCP imports/registration in server.py and tasks.py
- Replace ai-copilot/mcp/ai-support extras with single ai-features extra
- Add stub MCP routes returning 501 when MCP is not installed
- Add gns3server-uninstall-ai-features CLI command
- Remove old gns3server-uninstall-ai-copilot command
- Update all error messages and docs to reference ai-features

Closes #2794

gns3server/api/routes/controller/__init__.py

@@ -39,7 +39,7 @@ else:
     async def ai_not_available(path: str = ""):
         raise HTTPException(
             status_code=status.HTTP_501_NOT_IMPLEMENTED,
-            detail="AI Copilot is not available. Install AI dependencies with: pip install gns3-server[ai-copilot]"
+            detail="AI Copilot is not available. Install AI dependencies with: pip install gns3-server[ai-features]"
         )
 
 from . import controller