GH-50478: [C++][Compute] Add utf8_view ignore_case test guarding UTF-8/Latin1 distinction

Add a match_substring ignore_case=true case on utf8_view with non-ASCII
input (uppercase E-acute) mirroring the existing MatchSubstringIgnoreCase
coverage. utf8_view is registered as StringViewType rather than the generic
BinaryViewType dispatch, so case folding spans the full Unicode range; this
test fails if that distinction is ever lost. Requested in review by
zanmato1984.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: fangchenli

cpp/src/arrow/compute/kernels/scalar_string_test.cc

@@ -2769,6 +2769,16 @@ TEST(TestStringViewPredicates, MatchLike) {
                      "[true, true, null, false, false]", &prefix);
   }
 }
+
+// utf8_view registers as StringViewType, so ignore_case folds the full Unicode
+// range (É matches é), not just ASCII. This would fail if utf8_view were
+// dispatched through the generic BinaryViewType path and lost that distinction.
+TEST(TestStringViewPredicates, MatchSubstringIgnoreCase) {
+  MatchSubstringOptions options{"aé(", /*ignore_case=*/true};
+  CheckScalarUnary("match_substring", utf8_view(),
+                   R"(["abc", "aEb", "baÉ(", "aé(", "ae(", "Aé("])", boolean(),
+                   "[false, false, true, true, false, true]", &options);
+}
 #endif
 
 TEST(TestStringViewPredicates, Utf8Length) {