load_mxlibrary: support multi-cell stencil icons (e.g. OCI Library.xml)

Problem

 `load_mxlibrary()` fails to parse icons from Oracle's official **OCI Architecture Diagram Toolkit** (`OCI Library.xml`,        
 224 icons). All 224 items return `"No valid mxCell found in XML"`.

 The issue is that OCI icons are **multi-cell Visio stencils** — each icon is an `mxGraphModel` containing 4-15 nested
 `mxCell` elements with `shape=stencil(...)` data. The current parser expects a single `mxCell` per library item.

 ## Reproduction

 ```python
 import drawpyo

 # This works (Azure icons are single-cell):
 drawpyo.register_mxlibrary(
     "azure",
     "https://raw.githubusercontent.com/dwarfered/azure-architecture-icons-for-drawio/main/azure-public-service-icons/004%2     
 0azure%20ecosystem.xml"
 )

 # This fails (OCI icons are multi-cell):
 shapes = drawpyo.load_mxlibrary("OCI Library.xml")  # 0 shapes found
 # WARNING: No valid mxCell found in XML (224 errors)
 ```

 ## Root Cause

 Each OCI library item's decoded XML contains multiple mxCells with parent hierarchy:

 ```xml
 <!-- Azure icon (works): single mxCell -->
 <mxGraphModel><root>
   <mxCell id="0"/><mxCell id="1" parent="0"/>
   <mxCell id="2" style="shape=mxgraph.azure..." vertex="1" parent="1">
     <mxGeometry width="50" height="50"/>
   </mxCell>
 </root></mxGraphModel>

 <!-- OCI icon (fails): 5 mxCells with parent-child nesting -->
 <mxGraphModel><root>
   <mxCell id="0"/><mxCell id="1" parent="0"/>
   <mxCell id="2" style="fillColor=none;strokeColor=none;" parent="1">
     <mxGeometry x="10" width="84" height="73"/>
   </mxCell>
   <mxCell id="3" style="fillColor=#FFFFFF;shape=stencil(lVbb...);" parent="2">
     <mxGeometry width="82" height="71"/>
   </mxCell>
   <mxCell id="4" style="fillColor=#FFFFFF;shape=stencil(pVnJ...);" parent="2">
     <mxGeometry width="84" height="73"/>
   </mxCell>
   <mxCell id="5" style="fillColor=#2d5967;shape=stencil(nZRB...);" parent="2">
     <mxGeometry x="1" y="1" width="80" height="69"/>
   </mxCell>
   <mxCell id="6" style="verticalAlign=middle;shape=stencil(nZBL...);" parent="1">
     <mxGeometry y="77" width="104" height="40"/>
   </mxCell>
 </root></mxGraphModel>
 ```

 ## Expected Behavior

 `load_mxlibrary()` should handle multi-cell items — return all cells as a group/composite, or at minimum not silently
 discard them.

 ## Impact

 OCI Library is Oracle's official draw.io icon set (224 icons), downloaded from:
 https://docs.oracle.com/en-us/iaas/Content/General/Reference/graphicsfordiagrams.htm

 Other vendor libraries may have similar multi-cell structures.