insight-chat: code-review polish on the chat system_prompt override

- Trim the override input once via Option::map(str::trim).filter(...).
- Use matches!() in restore_system_prompt_override's Prepended arm so
  it reads consistently with the Replaced arm.

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

src/ai/insight_chat.rs

@@ -1205,10 +1205,7 @@ pub(crate) fn apply_system_prompt_override(
     messages: &mut Vec<ChatMessage>,
     override_prompt: Option<&str>,
 ) -> Option<SystemPromptStash> {
-    let prompt = match override_prompt {
-        Some(s) if !s.trim().is_empty() => s.trim().to_string(),
-        _ => return None,
-    };
+    let prompt = override_prompt.map(str::trim).filter(|s| !s.is_empty())?.to_string();
     if let Some(first) = messages.first_mut()
         && first.role == "system"
     {
@@ -1235,7 +1232,7 @@ pub(crate) fn restore_system_prompt_override(
             }
         }
         SystemPromptStash::Prepended => {
-            if !messages.is_empty() && messages[0].role == "system" {
+            if matches!(messages.first(), Some(m) if m.role == "system") {
                 messages.remove(0);
             }
         }