fix(com): create linear dimensions with AddDimRotated (#3)

dimension_linear always failed with '<unknown>.AddDimLinear' on the COM
backend — pywin32's dynamic dispatch error for a method that does not
exist: the AutoCAD ActiveX API has no AddDimLinear. AddDimRotated takes
the same arguments (XLine1Point, XLine2Point, DimLineLocation,
RotationAngle), so the call site only changes the method name.

Adds a mocked regression test asserting AddDimRotated is called with
the exact point/rotation arguments and AddDimLinear never is.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backends/com_backend.py

@@ -824,7 +824,9 @@ class ComBackend(AutoCADBackend):
     ) -> EntityInfo:
         def _sync():
             mspace = _msp()
-            dim = mspace.AddDimLinear(
+            # The ActiveX API has no AddDimLinear — rotated linear dims are
+            # created with AddDimRotated (GH issue #3).
+            dim = mspace.AddDimRotated(
                 _apoint(x1, y1), _apoint(x2, y2),
                 _apoint(dim_x, dim_y), deg2rad(rotation),
             )