Fix flaky CI tests caused by matplotlib font cache warning

The matplotlib font cache warning message was appearing in CI environments
on first run, causing test_gated_solution_build and test_gated_solution_doctree
to fail when comparing output with regression fixtures.

Added regex patterns to FileRegression.changes to strip out the matplotlib
font cache stderr output in both HTML and XML doctree formats.

Author: mmcky

tests/conftest.py

@@ -99,6 +99,15 @@ class FileRegression:
         (r"ipykernel_\d+", "ipykernel_XXXXX"),
         # Normalize matplotlib image hashes (platform/version dependent)
         (r"[a-f0-9]{64}\.png", "IMAGEHASH.png"),
+        # Strip matplotlib font cache warning (appears in CI on first run)
+        (
+            r'<div class="output stderr highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Matplotlib is building the font cache; this may take a moment\.\n</pre></div>\n</div>\n',
+            "",
+        ),
+        (
+            r'<literal_block classes="output stderr" language="myst-ansi" xml:space="preserve">\s*Matplotlib is building the font cache; this may take a moment\.\n\s*',
+            "",
+        ),
     )
 
     def __init__(self, file_regression):