Use len(draw_data.cmd_lists) in imgui backend

<details><summary>Claude's draft</summary>

`ImDrawData.cmd_lists_count` was a legacy alias, documented in the imgui_bundle
stubs as `== CmdLists.Size`, and imgui_bundle removed it in 1.92.900. Every
`ImguiWgpuBackend.render()` call raises AttributeError against that release.
Fixes #829.

`len(draw_data.cmd_lists)` is exactly what the removed attribute equalled, and
works across the whole supported `imgui-bundle>=1.92.0,<2` range: verified
against 1.92.0, 1.92.3, 1.92.4, 1.92.5, 1.92.600, 1.92.601, 1.92.700, 1.92.801
and 1.92.900. No shim or version check is needed, and the pin does not need
narrowing.

The test from the previous commit fails without this change and passes with it.

Resume this Claude session:
```
cd /home/mark/git/wgpu-py
claude --resume 590dd7bc-312c-419e-ad3d-2c69d00943cd
```
</details>

Author: hmaarrfk

CHANGELOG.md

@@ -33,6 +33,10 @@ Possible sections in each release:
   Downstream code should capture that error and skip the frame, optionally invoking some sort of sleep to save energy.
   See https://github.com/pygfx/wgpu-py/pull/820 for context.
 
+### Fixed:
+* The imgui backend no longer uses ``ImDrawData.cmd_lists_count``, which was removed in imgui-bundle 1.92.900.
+  See https://github.com/pygfx/wgpu-py/issues/829.
+
 
 ## [v0.31.1] - 23-06-2026
 

wgpu/utils/imgui/imgui_backend.py

@@ -445,7 +445,7 @@ def render(
         fb_width = int(display_width * draw_data.framebuffer_scale.x)
         fb_height = int(display_height * draw_data.framebuffer_scale.y)
 
-        if fb_width <= 0 or fb_height <= 0 or draw_data.cmd_lists_count == 0:
+        if fb_width <= 0 or fb_height <= 0 or len(draw_data.cmd_lists) == 0:
             return
 
         if draw_data.textures is not None: