fix(task-2): block_explode writes Normal as a VARIANT and keeps constant attributes as TEXT

COM: ``text.Normal`` was a plain Python tuple, which ActiveX marshals as
VT_ARRAY|VT_VARIANT and refuses with E_INVALIDARG (verified live on
AutoCAD 2026); because the write came after Explode(), the ATTDEF deletes
and AddText, the tool raised and left the drawing torn. It now goes
through ``_apoint`` like every other point write, and the fake ActiveX
surface refuses a bare tuple on Normal/InsertionPoint/TextAlignmentPoint
so a test cannot pin the wrong call shape again.

COM: every AcDbAttributeDefinition Explode() returned was deleted, but
GetAttributes() excludes constant attributes, so a constant attribute's
only visible text was destroyed with ok: True. A constant ATTDEF from the
explode (Constant member, or its tag in GetConstantAttributes()) is now
captured like an ATTRIB and replaced by a TEXT; placeholders are still
just deleted.

ezdxf: an AutoCAD-authored reference carries no ATTRIB for a constant
attribute and virtual_entities() skips ATTDEF, so the same text vanished
headlessly. A constant ATTDEF whose tag has no attached ATTRIB becomes a
TEXT transformed by the reference matrix, the path add_auto_attribs takes,
and is not converted twice when block_insert did attach one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

server.py

@@ -3031,8 +3031,9 @@ async def block_explode(
 
     Every attached ATTRIB becomes a TEXT with the same value, placement,
     height, rotation and layer (an invisible attribute becomes an invisible
-    TEXT); AutoCAD's plain EXPLODE would keep only the tag-name placeholders
-    and drop the values. Everything lands in the layout that owns the
+    TEXT), and a constant attribute (which has no ATTRIB) becomes a TEXT of
+    its value too; AutoCAD's plain EXPLODE would keep only the tag-name
+    placeholders and drop the values. Everything lands in the layout that owns the
     reference (model space or its paper-space sheet), whichever tab is
     current. Returns `inserted_handles` (the geometry), `attribute_texts`
     (one TEXT handle per attribute), `exploded_handle` and `backend`.