fix(.editorconfig): restore CA1052 suggestion severity

Cherry-pick dfd7ac3 ("Fix CA1052 in the codebase") removed the
CA1052 severity downgrade in src/Microsoft.DotNet.Wpf/src/.editorconfig
and converted some static-holder types to actually be static. But the
upstream PR (dotnet#10718) is still OPEN — h3xds1nz never finished fixing
all violations. Three types still trigger CA1052 in our cherry-picked
state:

  - MarshalGeneric<T>  (Marshalers.cs L339)  — base class, not
    convertible to static; would need to become `abstract`
  - XamlParser        (XamlParser.cs L58)   — constructors guarded
    by #if PBTCOMPILER; non-PBT build has only statics
  - XamlReaderHelper  (XamlReaderConstants.cs L28) — partial class

upstream/main still has `dotnet_diagnostic.CA1052.severity = suggestion`
in this same .editorconfig — so we match upstream's current state.

Properly finishing upstream PR 10718 (making the three types static/
sealed/abstract correctly) is queued for a follow-up — see FORK.md
"Conflicted PRs" list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Claude (Initial Force WPF Bot)

src/Microsoft.DotNet.Wpf/src/.editorconfig

@@ -17,6 +17,15 @@ csharp_style_pattern_matching_over_as_with_null_check = true:suggestion
 # CA1036: Override methods on comparable types
 dotnet_diagnostic.CA1036.severity = suggestion
 
+# CA1052: Static holder types should be Static or NotInheritable
+# Upstream PR #10718 (cherry-picked as dfd7ac3f6) removed this severity
+# downgrade, but the upstream PR is still OPEN and incomplete — it does
+# not fix MarshalGeneric<T>, XamlParser, or XamlReaderHelper. Reinstate
+# the suggestion-level severity so the build passes; finishing upstream
+# PR 10718 (making those three types static/sealed/abstract correctly)
+# is tracked as a follow-up.
+dotnet_diagnostic.CA1052.severity = suggestion
+
 # NOTE: If existing public API, CA1066 should be suppressed individually
 # in the GlobalSuppressions file.