`Symbol.iterator in FieldTree` is always false

### Which @angular/* package(s) are the source of the bug?

forms

### Is this a regression?

No

### Description

I want to traverse a `FieldTree`. A typesafe check for iterability requires checking for presence of `Symbol.iterator` field:

```ts
if (Symbol.iterator in fieldTree && typeof fieldTree[Symbol.iterator] === 'function') {
for (const [k, v] of fieldTree) {
// ...
}
}
```

However the field proxy doesn't implement `has` handler, therefore `Symbol.iterator` is reported as missing, even though it's returned by `get` handler:
https://github.com/angular/angular/blob/46d2cb7ff049dc4549080f64dceb1541b3a87cee/packages/forms/signals/src/field/proxy.ts#L67

### Please provide a link to a minimal reproduction of the bug

_No response_

### Please provide the exception or error you saw

```true

```

### Please provide the environment you discovered this bug in (run `ng version`)

```true
"@angular/forms": "^22.1.0",
```

### Anything else?

_No response_

Start in packages/forms/signals/src/field/proxy.ts at the linked proxy handler and inspect how FieldTree responds to property-presence checks versus property access. Verify that the provided TypeScript example recognizes Symbol.iterator and can traverse the FieldTree, while preserving the existing iterator behavior.

Written by the indexing model from the issue text.