Make test_examples_run actually draw a frame

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

test_examples_run() ran an example's main block and stopped there. Under the
offscreen canvas that only builds the canvas and registers a draw function —
nothing is drawn — so the draw function, which is the bulk of most examples,
was never executed. That is how #829 slipped through CI: the imgui examples
passed while ImguiWgpuBackend.render() was never entered.

Draw one frame after running the main block, for every example that exposes a
canvas. Also assert nothing was logged to the "rendercanvas" logger, because
rendercanvas calls the draw function inside its log_exception() context: an
exception there is logged and swallowed, and a frame still comes out, so
drawing without checking the log would prove nothing.

This drops the separate tests/test_util_imgui.py and the ci.yml change that
went with it; the existing `pytest -v examples` job already installs
imgui_bundle and now covers the backend.

Verified with imgui_bundle 1.92.900: both imgui examples fail without the
backend fix and pass with it, and the whole examples suite passes on 1.92.0
and 1.92.900.

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

Author: hmaarrfk

.github/workflows/ci.yml

@@ -124,9 +124,6 @@ jobs:
         EXPECT_LAVAPIPE: true
       run: |
           pytest -v examples
-          # Separate invocation: examples/tests and tests both have a
-          # testutils.py, so collecting them together shadows one of them.
-          pytest -v tests/test_util_imgui.py
 
   test-pyinstaller-build:
     name: Test PyInstaller

examples/tests/test_examples.py

@@ -4,6 +4,7 @@
 
 import os
 import importlib
+import logging
 import runpy
 import sys
 from unittest.mock import patch
@@ -177,11 +178,24 @@ def update_diffs(module, is_similar, img, stored_img, *, atol):
 
 
 @pytest.mark.parametrize("module", examples_to_run)
-def test_examples_run(module, force_offscreen):
+def test_examples_run(module, force_offscreen, caplog):
     """Run every example marked to see if they can run without error."""
-    # use runpy so the module is not actually imported (and can be gc'd)
-    # but also to be able to run the code in the __main__ block
-    runpy.run_module(f"examples.{module}", run_name="__main__")
+    with caplog.at_level(logging.ERROR, logger="rendercanvas"):
+        # use runpy so the module is not actually imported (and can be gc'd)
+        # but also to be able to run the code in the __main__ block
+        module_globals = runpy.run_module(f"examples.{module}", run_name="__main__")
+
+        # The main block only sets up the canvas and hands it a draw function;
+        # the offscreen canvas does not draw until asked. So ask, otherwise the
+        # draw function (the bulk of most examples) is never executed.
+        canvas = module_globals.get("canvas")
+        if canvas is not None:
+            canvas.draw()
+
+    # rendercanvas calls the draw function inside a log_exception() context, so
+    # errors in it do not propagate; they only show up in the log.
+    errors = [r for r in caplog.records if r.name == "rendercanvas"]
+    assert not errors, caplog.text
 
 
 if __name__ == "__main__":