gpui: Only time out multi-stroke bindings when current prefix matches (#42659)

Part One for Resolving #10910

### Summary
Typing prefix (partial keybinding) will behave like Vim. No timeout
until you either finish the sequence or hit Escape, while ambiguous
sequences still auto-resolve after 1s.

### Description
This follow-up tweaks the which-key system first part groundwork so our
timeout behavior matches Vim’s expectations. Then we can implement the
UI part in the next step (reference latest comments in
https://github.com/zed-industries/zed/pull/34798)
- `DispatchResult` now reports when the current keystrokes are already a
complete binding in the active context stack (`pending_has_binding`). We
only start the 1s flush timer in that case. Pure prefixes or sequences
that only match in other contexts—stay pending indefinitely, so
leader-style combos like `space f g` no longer evaporate after a second.
- `Window::dispatch_key_event` cancels any prior timer before scheduling
a new one and only spawns the background flush task when
`pending_has_binding` is true. If there’s no matching binding, we keep
the pending keystrokes and rely on an explicit Escape or more typing to
resolve them.

Release Notes:

- Fixed multi-stroke keybindings so only ambiguous prefixes auto-trigger
after 1 s; unmatched prefixes now stay pending until canceled, matching
Vim-style leader behavior.

crates/collab/src/tests/integration_tests.rs

@@ -6551,12 +6551,12 @@ async fn test_pane_split_left(cx: &mut TestAppContext) {
         assert!(workspace.items(cx).collect::<Vec<_>>().len() == 2);
     });
     cx.simulate_keystrokes("cmd-k");
-    // sleep for longer than the timeout in keyboard shortcut handling
-    // to verify that it doesn't fire in this case.
+    // Sleep past the historical timeout to ensure the multi-stroke binding
+    // still fires now that unambiguous prefixes no longer auto-expire.
     cx.executor().advance_clock(Duration::from_secs(2));
     cx.simulate_keystrokes("left");
     workspace.update(cx, |workspace, cx| {
-        assert!(workspace.items(cx).collect::<Vec<_>>().len() == 2);
+        assert!(workspace.items(cx).collect::<Vec<_>>().len() == 3);
     });
 }