Add support for "..." in edit_predictions.disabled_globs (#64540)

## Objective

Let users and projects add edit prediction exclusion patterns without
repeating defaults or inherited patterns.

Part of #64399.

## Solution

Extend the `"..."` convention from #62769, #64222, #64440, and #64484 to
`edit_predictions.disabled_globs` using `SplicingVec`. Inherited
patterns are inserted at `"..."`, and duplicates retain their first
occurrence. Omitting the marker replaces the list, `[]` clears it, and
omitting the setting preserves inheritance.

Opening an absent setting now initializes it with `["..."]` instead of
clearing inherited patterns, and the selection is located by parsing the
JSON instead of a regex, so a commented-out or nested `"disabled_globs"`
can no longer capture it. Cursor imports append `"..."` like the
`files.exclude` import does, so Zed's sensitive-file defaults survive an
import; empty lists and empty or literal `"..."` entries are dropped,
matching `enabled_patterns`.

## Testing

Tested on macOS with settings merge and loading tests, Cursor import
tests, the UI settings update and selection path (including
commented-out and nested decoy keys), the new `settings_json` key-path
lookup, and Copilot integration coverage.

To check manually, configure custom disabled patterns alongside `"..."`
and confirm that both custom patterns and inherited sensitive-file
patterns remain disabled. Repeat without `"..."` and with `[]` to verify
replacement and clearing. Opening the setting when it is absent should
preserve inherited patterns.

## Self-Review Checklist:

- [x] I’ve reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed’s UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Added support for `"..."` in `edit_predictions.disabled_globs` so
custom patterns can extend inherited exclusions, and fixed "Configure
Excluded Files" selecting the wrong list when a commented-out
`disabled_globs` precedes it.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/copilot/src/copilot_edit_prediction_delegate.rs

@@ -246,7 +246,7 @@ mod tests {
     use lsp::Uri;
     use project::Project;
     use serde_json::json;
-    use settings::{AllLanguageSettingsContent, DelayMs, SettingsStore};
+    use settings::{AllLanguageSettingsContent, DelayMs, SettingsStore, SplicingVec};
     use std::future::Future;
     use util::{
         path,
@@ -979,7 +979,7 @@ mod tests {
             settings
                 .edit_predictions
                 .get_or_insert(Default::default())
-                .disabled_globs = Some(vec![".env*".to_string()]);
+                .disabled_globs = Some(SplicingVec::from(vec![".env*".to_string()]));
         });
 
         let (copilot, copilot_lsp) = Copilot::fake(cx);