assistant: Improve terminal slash command (#16229)

- Fix terminal slash command not working when terminal tab was placed in
center workspace
- Removed `--line-count` argument, you can now just pass a number to the
slash command
e.g. `/terminal 10` will show the last 10 lines of the active terminal
- Increase default context lines to 50
- We will revisit this once we add support for only including the last n
amount of commands that were run

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -65,7 +65,7 @@ actions!(
     ]
 );
 
-const DEFAULT_CONTEXT_LINES: usize = 20;
+const DEFAULT_CONTEXT_LINES: usize = 50;
 
 #[derive(Clone, Default, Deserialize, PartialEq)]
 pub struct InlineAssist {