debugger: Fix bug where active debug line highlights weren't cleared (#29562)

## Context
The bug occurred because we stopped propagating the
`BreakpointStoreEvent::SetDebugLine` whenever a new debug line highlight
had been set. This was done to prevent multiple panes from having
editors focus on the debug line. However, it stopped the event from
propagating to editors that needed to clear their debug line highlights.

I fixed this by introducing two phases
1. Clear all debug line highlights
2. Set active debug line highlight in singular editor 

I also added a test to prevent regressions from occurring

Release Notes:

- N/A

crates/workspace/src/workspace.rs

@@ -998,7 +998,7 @@ impl Workspace {
                 | BreakpointStoreEvent::BreakpointsCleared(_) => {
                     workspace.serialize_workspace(window, cx);
                 }
-                BreakpointStoreEvent::ActiveDebugLineChanged => {}
+                BreakpointStoreEvent::SetDebugLine | BreakpointStoreEvent::ClearDebugLines => {}
             },
         )
         .detach();