fix(tests): force UTF-8 encoding when writing non-ASCII SKILL.md fixture (#979)

test_non_ascii_frontmatter_warning fails on Windows runners because
Path.write_text() defaults to the system locale (cp1252 on
windows-latest). The non-ASCII description bytes get mangled on disk,
the validator fails to decode as UTF-8, and the test sees a hard error
instead of the intended warning.

Pin encoding='utf-8' so bytes-on-disk are valid UTF-8 on every platform,
which exercises the actual non-ASCII warning code path the test asserts.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danielmeppiel

tests/unit/test_skill_bundle.py

@@ -226,7 +226,8 @@ def test_non_ascii_frontmatter_warning(self, tmp_path):
         sd = skills_dir / "unicode-skill"
         sd.mkdir()
         (sd / "SKILL.md").write_text(
-            "---\nname: unicode-skill\ndescription: Ünïcödé description\n---\n# x\n"
+            "---\nname: unicode-skill\ndescription: Ünïcödé description\n---\n# x\n",
+            encoding="utf-8",
         )
         result = validate_apm_package(tmp_path)
         assert result.is_valid  # warnings don't fail validation