fix(tui): prevent panic in HelpBrowserState when sections empty

Changed current_section() to return Option<&HelpSection> and use
safe .get() access instead of direct indexing. Updated render_content()
to handle None gracefully by early returning, and updated tests to use
.expect() for assertions.

Fixes #5149

Author: echobt

src/cortex-tui/src/widgets/help_browser/render.rs

@@ -152,7 +152,9 @@ impl<'a> HelpBrowser<'a> {
 
     /// Renders the content pane.
     fn render_content(&self, area: Rect, buf: &mut Buffer) {
-        let section = self.state.current_section();
+        let Some(section) = self.state.current_section() else {
+            return;
+        };
         let mut y = area.y;
         let scroll = self.state.content_scroll;
         let mut line_idx = 0;

src/cortex-tui/src/widgets/help_browser/state.rs

@@ -148,8 +148,13 @@ impl HelpBrowserState {
     }
 
     /// Returns the currently selected section.
-    pub fn current_section(&self) -> &HelpSection {
-        &self.sections[self.selected_section]
+    ///
+    /// Returns `None` if the sections vector is empty.
+    pub fn current_section(&self) -> Option<&HelpSection> {
+        if self.sections.is_empty() {
+            return None;
+        }
+        self.sections.get(self.selected_section)
     }
 
     /// Handles character input for search.

src/cortex-tui/src/widgets/help_browser/tests.rs

@@ -43,7 +43,7 @@ mod tests {
     #[test]
     fn test_help_browser_state_with_topic() {
         let state = HelpBrowserState::new().with_topic(Some("keyboard"));
-        assert_eq!(state.current_section().id, "keyboard");
+        assert_eq!(state.current_section().expect("should have section").id, "keyboard");
     }
 
     #[test]
@@ -220,10 +220,10 @@ mod tests {
     #[test]
     fn test_current_section() {
         let mut state = HelpBrowserState::new();
-        assert_eq!(state.current_section().id, "getting-started");
+        assert_eq!(state.current_section().expect("should have section").id, "getting-started");
 
         state.select_next();
-        assert_eq!(state.current_section().id, "keyboard");
+        assert_eq!(state.current_section().expect("should have section").id, "keyboard");
     }
 
     #[test]
@@ -232,4 +232,11 @@ mod tests {
         assert!(!state.sections.is_empty());
         assert_eq!(state.selected_section, 0);
     }
-}
+
+    #[test]
+    fn test_current_section_empty_sections() {
+        let mut state = HelpBrowserState::new();
+        state.sections.clear();
+        assert!(state.current_section().is_none());
+    }
+}