chore: Fix several style lints (#17488)

It's not comprehensive enough to start linting on `style` group, but
hey, it's a start.

Release Notes:

- N/A

crates/language_model/src/request.rs

@@ -212,7 +212,7 @@ impl LanguageModelRequestMessage {
         self.content.is_empty()
             || self
                 .content
-                .get(0)
+                .first()
                 .map(|content| match content {
                     MessageContent::Text(text) => text.trim().is_empty(),
                     MessageContent::ToolResult(tool_result) => {