settings: Fix VS Code import appending duplicate file associations (#61355)

Closes #56536

## What & why

Running `zed: import vs code settings` more than once kept re-adding the
same file extensions to `file_types`, so `files.associations` entries
grew without bound:

```json
"file_types": { "c": ["*.keymap", "*.keymap", "*.keymap"] }
```

The import merges the VS Code-derived settings into the user's existing
settings, and `file_types` values were `ExtendingVec`s, whose
`merge_from` appends every incoming value unconditionally.

Per review feedback, instead of changing `ExtendingVec`'s merge
semantics (it also backs ssh/wsl connections, `private_files`, etc.),
this adds a new `ExtendingSet` type that actually behaves like a set,
and switches `FileTypeMap`'s file associations to it:

- Backed by an insertion-order-preserving `IndexSet`, so users' pattern
order round-trips through the settings file unchanged (the import
machinery diffs serialized old/new content to produce edits; a sorted
set would reorder existing arrays).
- Merging only accumulates new values, so re-importing an
already-present association is a no-op.
- Since it's a real set, deserialization also collapses duplicates that
earlier imports already wrote into the settings file.
- `ExtendingVec` and all its other users are untouched.

## Testing

- Added a `test_vscode_import` case that re-imports an already-present
`files.associations` entry and asserts the extension isn't duplicated.
It fails before the change and passes after.
- `cargo test -p settings -p settings_content -p language` pass.
- `./script/clippy -p settings -p settings_content -p language -p
recent_projects -p json_schema_store` is clean.

Release Notes:

- Fixed importing VS Code settings repeatedly adding duplicate
`file_types` entries for `files.associations`.

---------

Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>

crates/settings/src/settings_store.rs

@@ -36,7 +36,7 @@ use crate::{
     LanguageToSettingsMap, LspSettings, LspSettingsMap, SemanticTokenRules, ThemeName,
     UserSettingsContentExt, VsCodeSettings, WorktreeId,
     settings_content::{
-        ExtendingVec, ExtensionsSettingsContent, ProfileBase, ProjectSettingsContent,
+        ExtendingSet, ExtensionsSettingsContent, ProfileBase, ProjectSettingsContent,
         RootUserSettings, SettingsContent, UserSettingsContent, merge_from::MergeFrom,
     },
 };
@@ -1203,7 +1203,7 @@ impl SettingsStore {
             });
 
             let file_type_patterns_ref =
-                generator.subschema_for::<ExtendingVec<String>>().to_value();
+                generator.subschema_for::<ExtendingSet<String>>().to_value();
             replace_subschema::<FileTypeMap>(generator, || {
                 json_schema!({
                     "type": "object",
@@ -2616,6 +2616,32 @@ mod tests {
             .unindent(),
             cx,
         );
+
+        // re-importing a file association that is already present should be
+        // idempotent rather than appending a duplicate extension (#56536)
+        check_vscode_import(
+            &mut store,
+            r#"{
+              "file_types": {
+                "c": ["*.keymap"]
+              }
+            }
+            "#
+            .unindent(),
+            r#"{ "files.associations": { "*.keymap": "c" } }"#.to_owned(),
+            r#"{
+              "base_keymap": "VSCode",
+              "minimap": {
+                "show": "always"
+              },
+              "file_types": {
+                "c": ["*.keymap"]
+              }
+            }
+            "#
+            .unindent(),
+            cx,
+        );
     }
 
     #[track_caller]

crates/settings/src/vscode_import.rs

@@ -644,11 +644,15 @@ impl VsCodeSettings {
 
     fn file_types(&self) -> Option<FileTypeMap> {
         // vscodes file association map is inverted from ours, so we flip the mapping before merging
-        let mut associations: HashMap<Arc<str>, ExtendingVec<String>> = HashMap::default();
+        let mut associations: HashMap<Arc<str>, ExtendingSet<String>> = HashMap::default();
         let map = self.read_value("files.associations")?.as_object()?;
         for (k, v) in map {
             let Some(v) = v.as_str() else { continue };
-            associations.entry(v.into()).or_default().0.push(k.clone());
+            associations
+                .entry(v.into())
+                .or_default()
+                .0
+                .insert(k.clone());
         }
         skip_default(FileTypeMap(associations))
     }