fix: Fix expression editor decoration (#4261)

## Description

2 bugs

### Click on test broke decoration


![image](https://github.com/user-attachments/assets/43a98ea1-08bd-4044-92ff-0a3735dfd6dc)

![image](https://github.com/user-attachments/assets/49d577d1-fa8f-40a1-9798-1fd41b5eaf40)


### Symbols after decoration broke decoration


![image](https://github.com/user-attachments/assets/27f96aa4-f512-44fb-99c8-604817b4dbf8)

![image](https://github.com/user-attachments/assets/93eff6b4-1e40-430e-a406-99b0bdcecd90)


### Would be good to improve

Error messages:

<img width="534" alt="image"
src="https://github.com/user-attachments/assets/a42509e6-8e41-40d3-b1ae-ad739487c68e">


## Steps for reproduction

1. click button
2. expect xyz

## Code Review

- [ ] hi @kof, I need you to do
  - conceptual review (architecture, feature-correctness)
  - detailed review (read every line)
  - test it on preview

## Before requesting a review

- [ ] made a self-review
- [ ] added inline comments where things may be not obvious (the "why",
not "what")

## Before merging

- [ ] tested locally and on preview environment (preview dev login:
5de6)
- [ ] updated [test
cases](https://github.com/webstudio-is/webstudio/blob/main/apps/builder/docs/test-cases.md)
document
- [ ] added tests
- [ ] if any new env variables are added, added them to `.env` file

apps/builder/app/builder/shared/expression-editor.tsx

@@ -271,12 +271,31 @@ class VariableWidget extends WidgetType {
 
 const variableMatcher = new MatchDecorator({
   regexp: /(\$ws\$dataSource\$\w+)/g,
-  decoration: (match, view) => {
+
+  decorate: (add, from, _to, match, view) => {
+    const startPos = match.index;
+
     const name = match[1];
-    const [data] = view.state.facet(VariablesData);
-    return Decoration.replace({
-      widget: new VariableWidget(data.aliases.get(name) ?? name),
-    });
+    const [{ aliases }] = view.state.facet(VariablesData);
+
+    // The regexp may match variables not in scope, but the key problem we're solving is this:
+    // We have an alias $ws$dataSource$321 -> SomeVar, which we display as '[SomeVar]' ([] means decoration in the editor).
+    // If the user types a symbol (e.g., 'a') immediately after '[SomeVar]',
+    // the raw text becomes $ws$dataSource$321a, but we want to display '[SomeVar]a'.
+    const dataSourceId = [...aliases.keys()].find((key) => name.includes(key));
+
+    if (dataSourceId === undefined) {
+      return;
+    }
+    const endPos = startPos + dataSourceId.length;
+
+    add(
+      from,
+      endPos,
+      Decoration.replace({
+        widget: new VariableWidget(aliases.get(dataSourceId)!),
+      })
+    );
   },
 });