feat: make AI Copilot an optional dependency

This change makes AI Copilot features optional to reduce installation
size and support restricted environments.

Changes:
- Split AI dependencies into ai-requirements.txt
- Add ai-copilot optional dependency in pyproject.toml
- Add import protection in gns3server/agent/__init__.py
- Return 501 for AI endpoints when dependencies not installed
- Add gns3server-uninstall-ai-copilot command for cleanup
- Update README with installation and uninstallation instructions

Installation:
- Basic: pip install gns3-server
- With AI: pip install gns3-server[ai-copilot]
- Development: pip install gns3-server[ai-copilot,dev]

Uninstallation:
- gns3server-uninstall-ai-copilot

gns3server/utils/uninstall_ai_copilot.py

@@ -0,0 +1,92 @@
+#!/usr/bin/env python3
+"""
+Uninstall AI Copilot dependencies.
+
+Usage:
+    gns3server-uninstall-ai-copilot
+    gns3server-uninstall-ai-copilot -y
+"""
+
+import os
+import sys
+import subprocess
+import argparse
+
+
+def get_ai_packages():
+    """Read packages from ai-requirements.txt."""
+    # Find the requirements file
+    if hasattr(sys, '_MEIPASS'):
+        # PyInstaller bundle
+        base_dir = os.path.dirname(sys.executable)
+    else:
+        # __file__ = gns3server/utils/uninstall_ai_copilot.py
+        # Need to go up 2 levels to reach project root
+        base_dir = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+    requirements_file = os.path.join(base_dir, "ai-requirements.txt")
+
+    packages = []
+    with open(requirements_file, "r") as f:
+        for line in f:
+            line = line.strip()
+            if not line or line.startswith("#"):
+                continue
+            # Remove version specifiers
+            package = line.split(">=")[0].split("==")[0].split("~=")[0]
+            packages.append(package)
+    return packages
+
+
+def uninstall(packages, yes=False):
+    """Uninstall packages."""
+    if not packages:
+        print("No packages found to uninstall.")
+        return
+
+    print(f"Found {len(packages)} AI Copilot dependencies:")
+    for pkg in packages:
+        print(f"  - {pkg}")
+    print()
+
+    if not yes:
+        response = input("Do you want to uninstall these packages? [y/N]: ")
+        if response.lower() != "y":
+            print("Cancelled.")
+            return
+
+    print("Uninstalling...")
+    for package in packages:
+        try:
+            result = subprocess.run(
+                [sys.executable, "-m", "pip", "uninstall", "-y", package],
+                capture_output=True,
+                text=True
+            )
+            if result.returncode == 0:
+                print(f"  Removed: {package}")
+            else:
+                print(f"  Failed to remove {package} (may not be installed)")
+        except Exception as e:
+            print(f"  Error removing {package}: {e}")
+
+    print("\nAI Copilot dependencies have been uninstalled.")
+    print("You can reinstall them with: pip install gns3-server[ai-copilot]")
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description="Uninstall AI Copilot dependencies"
+    )
+    parser.add_argument(
+        "-y", "--yes", action="store_true",
+        help="Automatically confirm uninstallation"
+    )
+    args = parser.parse_args()
+
+    packages = get_ai_packages()
+    uninstall(packages, yes=args.yes)
+
+
+if __name__ == "__main__":
+    main()