Observer Can Be Added Without Phone Number

[FloBut]

### Describe the issue

When adding a new observer through the "Add individual observer" form, the system allows saving the observer without entering a phone number.

### Steps to reproduce

A box opens with a search loop (loading spinner).
Nothing can be typed or selected in the box.
A button labeled "Close" is displayed under the loop.
Pressing the "Close" button collapses the Tags field, and the user can continue by pressing "Add individual observer", but without being able to add any tags.
This means the Tags functionality cannot be used.

Steps to reproduce

1. Go to Monitoring Observer List.
2. Click Add individual observer.
3. Fill in the required fields (e.g., First Name, Last Name, Email).
4. Leave the Phone Number field empty.
5. Click Save.

Observe that the observer is successfully created, even though no phone number was provided.

_ ### Expected behavior_

The Phone Number field should be required.

The form should not allow saving an observer without entering a valid phone number.

A validation message should appear if the field is left empty.

### Environment

Browser: Chrome 114
OS: EMUI 12.0.0
Device: Huawei Mate 10 Pro

### Additional context