parser: port test "switch multiline string"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

parser_test.zig

@@ -2723,6 +2723,37 @@ test "zig fmt: nested struct literal with one item" {
     );
 }
 
+test "zig fmt: switch multiline string" {
+    try testCanonical(
+        \\test "switch multiline string" {
+        \\    const x: u32 = 0;
+        \\    const str = switch (x) {
+        \\        1 => "one",
+        \\        2 =>
+        \\        \\ Comma after the multiline string
+        \\        \\ is needed
+        \\        ,
+        \\        3 => "three",
+        \\        else => "else",
+        \\    };
+        \\
+        \\    const Union = union(enum) {
+        \\        Int: i64,
+        \\        Float: f64,
+        \\    };
+        \\
+        \\    const str = switch (u) {
+        \\        Union.Int => |int|
+        \\        \\ Comma after the multiline string
+        \\        \\ is needed
+        \\        ,
+        \\        Union.Float => |*float| unreachable,
+        \\    };
+        \\}
+        \\
+    );
+}
+
 test "zig fmt: switch cases trailing comma" {
     try testTransform(
         \\test "switch cases trailing comma"{