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

```

## 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.

Start at load_mxlibrary and reproduce the failure with OCI Library.xml, inspecting each decoded mxGraphModel/root and its parent-linked mxCell elements. Done means multi-cell library items are retained as a group or composite instead of being discarded, while preserving support for the existing single-cell libraries.

Written by the indexing model from the issue text.