fix: correct test assertion in dim_false_plain_text_has_no_color

The first assertion was checking span_text() which concatenates all spans
including trailing paragraph-end newlines, making the test guaranteed to fail.
Changed to check spans[0].content directly to verify the text span itself
contains 'hello world' with no color applied when dim=false.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/view/markdown.rs

@@ -444,7 +444,7 @@ mod tests {
     #[test]
     fn dim_false_plain_text_has_no_color() {
         let spans = render_markdown("hello world", 0, false);
-        assert_eq!(span_text(&spans), "hello world");
+        assert_eq!(spans[0].content.as_ref(), "hello world");
         assert_eq!(spans[0].style, Style::default());
     }