feat: add packet filter management tool for GNS3-Copilot fault injection

Add comprehensive packet filter management functionality to GNS3-Copilot,
enabling AI-powered fault injection scenarios with network simulation
capabilities like latency, packet loss, and corruption.

## Changes

### New Features
- **GNS3PacketFilterTool**: New LangChain tool for managing packet filters
  on GNS3 links with support for delay, packet loss, corruption,
  frequency_drop, and BPF filtering
  - Actions: get_available, set, get, clear
  - Integrated into troubleshooting_injection mode for fault scenarios

### API Integration
- **Link.available_filters()**: Added method to custom_gns3fy.py Link class
  - Queries available filter types for specific links
  - API v3+ only (raises ValueError for v2 connectors)
  - Returns filter definitions with parameters and constraints

### Tool Integration
- Added GNS3PacketFilterTool to TROUBLESHOOTING_INJECTION_MODE_TOOLS
- Positioned as 3rd tool in fault injection workflow
- Optimized for troubleshooting practice scenarios

## Files Modified
- gns3server/agent/gns3_copilot/agent/gns3_copilot.py
- gns3server/agent/gns3_copilot/gns3_client/custom_gns3fy.py
- gns3server/agent/gns3_copilot/tools_v2/__init__.py

## Files Added
- gns3server/agent/gns3_copilot/tools_v2/gns3_packet_filter.py

## Testing
- All validation tests passed
- Version checking verified (v3+ only)
- Tool integration confirmed in troubleshooting mode

gns3server/agent/gns3_copilot/agent/gns3_copilot.py

@@ -88,6 +88,7 @@ from gns3server.agent.gns3_copilot.tools_v2 import (
 )
 from gns3server.agent.gns3_copilot.tools_v2 import GNS3CreateNodeTool
 from gns3server.agent.gns3_copilot.tools_v2 import GNS3LinkTool
+from gns3server.agent.gns3_copilot.tools_v2 import GNS3PacketFilterTool
 from gns3server.agent.gns3_copilot.tools_v2 import GNS3StartNodeTool
 from gns3server.agent.gns3_copilot.tools_v2 import GNS3StopNodeTool
 from gns3server.agent.gns3_copilot.tools_v2 import GNS3SuspendNodeTool
@@ -144,6 +145,7 @@ LAB_AUTOMATION_ASSISTANT_MODE_TOOLS = [
 TROUBLESHOOTING_INJECTION_MODE_TOOLS = [
     ExecuteMultipleDeviceCommands(),  # Get device configurations (READ-ONLY)
     ExecuteMultipleDeviceConfigCommands(),  # Inject configuration changes
+    GNS3PacketFilterTool(),  # Manage packet filters on links (delay, loss, corrupt, etc.)
     InjectionSkillsTool(),  # Query injection skills and fault types
     GNS3TopologyTool(),  # Get topology information
 ]