refactor(mcp): drop the raw drawing_create tool

The templated drawing_create_text/_rectangle/_ellipse tools cover the
LLM-relevant creation paths, and line/image/curve creation is not
needed from MCP or copilot for now. The management tools stay, so
existing drawings (including images and lines created in the Web UI)
remain fully manageable; drawing_update's raw svg parameter remains
the escape hatch for edits.

tests/agent/mcp/test_tool_params.py

@@ -83,7 +83,6 @@ HANDLER_FILES = {
     "delete_snapshot_handler": "snapshots.py",
     "restore_snapshot_handler": "snapshots.py",
     "get_drawings_handler": "drawings.py",
-    "create_drawing_handler": "drawings.py",
     "get_drawing_handler": "drawings.py",
     "update_drawing_handler": "drawings.py",
     "delete_drawing_handler": "drawings.py",