feat(agent): share Web UI compatible drawing builders between MCP and copilot

The text drawing tool was MCP-only; the copilot needs the same
capability and its area-annotation helper carried the same Web UI
compat bugs. Split into three layers with a single implementation:

- gns3server/utils/drawings.py: pure SVG builders (text, rectangle,
  ellipse) with full input validation (ValueError) — the Web UI
  format knowledge lives in core utils next to picture.py
- api_handlers.py: shared REST handlers (create_text_drawing_handler
  moved here, create_rectangle/ellipse_drawing_handler added) — the
  layer both consumers already share
- thin shells: MCP tools (drawing_create_text unchanged, plus new
  drawing_create_rectangle/_ellipse) and three LangChain tools
  (tools_v2/gns3_create_drawing.py) registered in both copilot mode
  lists

Shape builder details:

- rectangle: solid fill or "none", stroke block emitted only when a
  stroke is given, rx corner radius, dasharray validated against the
  Web UI's Qt remap table (QtDasharrayFixer rewrites those patterns)
- ellipse: bounding-box API; cx/cy derived as offsets inside the
  drawing box (the Web UI treats them as box-relative, not canvas
  coordinates); odd sizes round down and the actual size is returned

gns3_drawing_utils now delegates to the shared builders, fixing the
px-blind sizing, the ignored text-anchor/x/y centering and the
missing XML escaping in the area-annotation path.

gns3server/agent/gns3_copilot/tools_v2/__init__.py

@@ -40,6 +40,7 @@ Main modules:
 - vpcs_tools_netmiko: VPCS device configuration tool using Netmiko
 - gns3_create_node: GNS3 node creation tool
 - gns3_create_link: GNS3 link creation tool
+- gns3_create_drawing: GNS3 canvas drawing tools (text, rectangle, ellipse)
 - gns3_start_node: GNS3 node startup tool (immediate return, no boot wait)
 - gns3_wait: Wait timer tool (pair with node start to let devices boot)
 - gns3_get_node_temp: GNS3 template retrieval tool
@@ -54,6 +55,11 @@ Author: Yue Guobin (岳国宾)
 # Import main tool classes
 from .config_tools_nornir import ExecuteMultipleDeviceConfigCommands
 from .display_tools_nornir import ExecuteMultipleDeviceCommands
+from .gns3_create_drawing import (
+    GNS3CreateEllipseDrawingTool,
+    GNS3CreateRectangleDrawingTool,
+    GNS3CreateTextDrawingTool,
+)
 from .gns3_create_link import GNS3LinkTool
 from .gns3_create_node import GNS3CreateNodeTool
 from .gns3_get_node_temp import GNS3TemplateTool
@@ -81,7 +87,10 @@ __url__ = "https://github.com/yueguobin/gns3-copilot"
 __all__ = [
     "ExecuteMultipleDeviceCommands",
     "ExecuteMultipleDeviceConfigCommands",
+    "GNS3CreateEllipseDrawingTool",
     "GNS3CreateNodeTool",
+    "GNS3CreateRectangleDrawingTool",
+    "GNS3CreateTextDrawingTool",
     "GNS3LinkTool",
     "GNS3PacketFilterTool",
     "GNS3StartNodeQuickTool",