Fix tasks not being stopped on reruns (#29786)

Follow-up of https://github.com/zed-industries/zed/pull/28993

* Tone down tasks' cancellation logging
* Fix task terminals' leak, disallowing to fully cancel the task by
dropping the terminal off the pane:

f619d5f02a/crates/terminal_view/src/terminal_panel.rs (L1464-L1471)

Release Notes:

- Fixed tasks not being stopped on reruns

crates/workspace/src/workspace.rs

@@ -136,7 +136,7 @@ pub trait TerminalProvider {
         task: SpawnInTerminal,
         window: &mut Window,
         cx: &mut App,
-    ) -> Task<Result<ExitStatus>>;
+    ) -> Task<Option<Result<ExitStatus>>>;
 }
 
 pub trait DebuggerProvider {