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

docs/development-setup.md

@@ -104,7 +104,7 @@ source venv/bin/activate
 # pip install -e . -i https://mirrors.aliyun.com/pypi/simple/
 
 pip install -e . && gns3server-web-wireshark-setup
-pip install -e .[ai-copilot]
+pip install -e .[ai-features]
 pip install -e .[dev]
 ```
 
@@ -114,10 +114,10 @@ Run the server:
 python3 -m gns3server
 ```
 
-## Optional: Install AI Copilot Development Dependencies
+## Optional: Install AI Features Development Dependencies
 
 ```bash
-python3 -m pip install .[ai-copilot,dev]
+python3 -m pip install .[ai-features,dev]
 ```
 
 ## Optional: Expand LVM Root Partition