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/schemas/__init__.py

@@ -30,24 +30,31 @@ from .controller.gns3vm import GNS3VM
 from .controller.nodes import NodeCreate, NodeUpdate, NodeDuplicate, NodeCapture, Node
 from .controller.projects import ProjectCreate, ProjectUpdate, ProjectDuplicate, Project, ProjectFile, ProjectCompression
 from .controller.users import UserCreate, UserUpdate, LoggedInUserUpdate, User, Credentials, UserGroupCreate, UserGroupUpdate, UserGroup
-from .controller.llm_model_configs import (
-    LLMModelConfigData,
-    LLMModelConfigCreate,
-    LLMModelConfigUpdate,
-    LLMModelConfigResponse,
-    LLMModelConfigWithSource,
-    LLMModelConfigInheritedResponse,
-    LLMModelConfigListResponse
-)
-from .controller.chat import (
-    OpenAIToolCall,
-    ChatRequest,
-    ChatResponse,
-    OpenAIMessage,
-    ConversationHistory,
-    ChatSession,
-    RenameSession
-)
+
+# Conditionally import AI-related schemas
+try:
+    from .controller.llm_model_configs import (
+        LLMModelConfigData,
+        LLMModelConfigCreate,
+        LLMModelConfigUpdate,
+        LLMModelConfigResponse,
+        LLMModelConfigWithSource,
+        LLMModelConfigInheritedResponse,
+        LLMModelConfigListResponse
+    )
+    from .controller.chat import (
+        OpenAIToolCall,
+        ChatRequest,
+        ChatResponse,
+        OpenAIMessage,
+        ConversationHistory,
+        ChatSession,
+        RenameSession
+    )
+except ImportError:
+    # AI schemas are not available (should not happen as they don't depend on external libs)
+    pass
+
 from .controller.rbac import RoleCreate, RoleUpdate, Role, Privilege, ACECreate, ACEUpdate, ACE
 from .controller.pools import Resource, ResourceCreate, ResourcePoolCreate, ResourcePoolUpdate, ResourcePool
 from .controller.tokens import Token