fix(forms): cleanup dynamically removed form fields

[Trandx]

Fix dynamic field cleanup in forms

Related issue: #8408

Problem

When dynamically removing form fields using v-if, v-for, or custom dynamic arrays, the field state was not properly cleaned up from the internal form store.

Although the component was destroyed, the following data remained registered internally:

• field state (_states)
• validation errors
• dirty/touched flags
• field watchers

This caused several issues:

• stale validation errors
• invalid form state after field removal
• orphan reactive watchers
• memory leaks
• removed fields still appearing in submit state

---

Example

<MyField v-if="visible" name="email" />

When visible becomes false, the component is unmounted visually, but the internal form state still contains:

_states.email

As a result:

• the form may still be invalid
• validation errors may still exist
• watchers continue reacting to changes

The same issue also affects dynamic arrays:

<MyField
  v-for="(item, index) in items"
  :key="item.id"
  :name="`contacts.${index}.email`"
/>

Removing an item from the array leaves stale field states and watchers behind.

---

Root Cause

defineField() registers field state and watchers internally, but there was no cleanup mechanism when a field component was unmounted.

As a result, dynamically removed fields remained inside:

fields
_states

---

Solution

This PR introduces a new unregister(field) cleanup method in useForm.

The method now:

• stops the field watcher
• removes the field definition
• clears the internal field state

Field components automatically call unregister() during beforeUnmount.

---

Result

Dynamic form fields are now properly cleaned up when removed.

This fixes:

• stale validation states
• lingering errors
• orphan watchers
• invalid form validity after field removal

and significantly improves dynamic form reliability.

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Test Reproduction

https://stackblitz.com/edit/vitejs-vite-pub96hsp