Missing diagnostic error when amending non-`Object` types

### Summary

When assigning a value of the wrong type to a `String`-typed property, the Pkl VS Code extension correctly flags a type mismatch *only* for primitive non‑string values (e.g., booleans), but **fails to flag the error when the incorrect value is an object literal**.

### Example

Given a field:
```pkl
my_property: String
```

This invalid assignment is correctly flagged:
```pkl
my_property = true // type mismatch is highlighted
```

But this invalid assignment is NOT flagged:
```pkl
my_property {
value = true
}
```
The second form assigns an object, which should not be allowed for a `String`-typed property, but the VS Code extension does not report any error.

### Expected Behavior
The language server should report a type mismatch when a `String`-typed property is assigned an object literal.

### Actual Behavior
No diagnostic is shown when assigning an object literal.

### Environment
Extension version: 0.21.0
Pkl version: 0.30.0
VS Code: latest stable

### Steps to Reproduce
1. Create a `.pkl` file with a `String` property.
1. Assign an object literal to it.
1. Observe that no type-checking error is shown.

### Additional Notes
This looks like a missing diagnostic case in the language server’s type checker.

Reproduce the issue in a .pkl file using a String-typed property and an object literal assignment, then trace the language server's type-checking path for object values. The work is done when the server reports a type-mismatch diagnostic for the object literal, matching the existing diagnostic for primitive values.

Written by the indexing model from the issue text.