fix(integrations): Incoming Webhook 'Avatar URL' field has no validation

Description:
When creating a new Incoming Webhook, the "Avatar URL" field does not have any client-side validation. It allows the user to save any invalid string (e.g., "jjjj"), even though the help text says, "Should be a URL of an image."

This can lead to a broken avatar icon for the integration's posts and user confusion.

Steps to reproduce:
Navigate to Administration > Integrations.

Click the "New" button in the top-right corner.

Select the "Incoming" tab (this is the default).

Fill in all required fields (e.g., Name: Test Avatar, Post to Channel: #general, Post as: Kalyan12).

Scroll down to the "Avatar URL" field.

Type an invalid string that is not a URL (e.g., jjjj or not-a-url).

Click the "Save" button at the bottom.

Expected behavior:
The form should not save. Because a value was provided, it should be validated. An error message (like a pop-up toast) should appear, stating "Please enter a valid URL."

If the field is left empty, it should save successfully.

Actual behavior:
The form saves successfully with the invalid string in the "Avatar URL" field. A green "Integration has been saved" toast message appears, and the integration is created with a broken avatar.

Server Setup Information:
Version of Rocket.Chat Server: develop (running from local source)

License Type: Community

Operating System: Ubuntu (via WSL)

Deployment Method: Local development (from source)

NodeJS Version: 22.16.0

MongoDB Version: 6.10.0

Client Setup Information
Desktop App or Browser Version: Chrome (latest)

Operating System: Windows

Additional context:

This appears to be a similar root cause as Issue #37411 (for Outgoing Webhooks). The IncomingWebhookForm.tsx (or its parent component) is missing a client-side validation rule (like a validate function or pattern) in react-hook-form for the avatar field.

Relevant logs:
No server or console logs are generated because the application incorrectly treats this as a successful operation. The bug is the absence of a validation error.

Start with IncomingWebhookForm.tsx or its parent component and reproduce the invalid Avatar URL submission using the steps in the issue. Check how the Avatar URL field is registered and how save errors are displayed. Done means invalid non-empty values are rejected with a validation message while an empty field can still be saved.