Fix bindings_for_action handling of shadowed key bindings (#32220)

Fixes two things:

* ~3 months ago [in PR
#26420](https://github.com/zed-industries/zed/pull/26420/files#diff-33b58aa2da03d791c2c4761af6012851b7400e348922d64babe5fd48ac2a8e60)
`bindings_for_action` was changed to return bindings even when they are
shadowed (when the keystrokes would actually do something else).

* For edit prediction keybindings there was some odd behavior where
bindings for `edit_prediction_conflict` were taking precedence over
bindings for `edit_prediction` even when the `edit_prediction_conflict`
predicate didn't match. The workaround for this was #24812. The way it
worked was:

    - List all bindings for the action

- For each binding, get the highest precedence binding with the same
input sequence

- If the highest precedence binding has the same action, include this
binding. This was the bug - this meant that if a binding in the keymap
has the same keystrokes and action it can incorrectly take display
precedence even if its context predicate does not pass.

- Fix is to check that the highest precedence binding is a full match.
To do this efficiently, it's based on an index within the keymap
bindings.

Also adds `highest_precedence_binding_*` variants which avoid the
inefficiency of building lists of bindings just to use the last.

Release Notes:

- Fixed display of keybindings to skip bindings that are shadowed by a
binding that uses the same keystrokes.
- Fixed display of `editor::AcceptEditPrediction` bindings to use the
normal precedence that prioritizes user bindings.

crates/gpui/src/platform/mac/platform.rs

@@ -315,6 +315,9 @@ impl MacPlatform {
                     action,
                     os_action,
                 } => {
+                    // Note that this is intentionally using earlier bindings, whereas typically
+                    // later ones take display precedence. See the discussion on
+                    // https://github.com/zed-industries/zed/issues/23621
                     let keystrokes = keymap
                         .bindings_for_action(action.as_ref())
                         .find_or_first(|binding| {