fix(ui): blockType ignored when merging server form state (#12207)

In this case, the `blockType` property is created on the server, but -
prior to this fix - was discarded on the client in
[`fieldReducer.ts`](https://github.com/payloadcms/payload/blob/main/packages/ui/src/forms/Form/fieldReducer.ts#L186-L198)
via
[`mergerServerFormState.ts`](b9832f40e4/packages/ui/src/forms/Form/mergeServerFormState.ts (L29-L31)),
because the field's path neither existed in the client's form state, nor
was it marked as `addedByServer`.

This caused later calls to POST requests to form state to send without
the `blockType` key for block rows, which in turn caused
`addFieldStatePromise.ts` to throw the following error:

```
Block with type "undefined" was found in block data, but no block with that type is defined in the config for field with schema path ${schemaPath}.
```

This prevented the client side form state update from completing, and if
the form state was saved, broke the document.

This is a follow-up to #12131, which treated the symptom, but not the
cause. The original issue seems to have been introduced in
https://github.com/payloadcms/payload/releases/tag/v3.34.0. It's unclear
to me whether this issue is connected to block E2E tests having been
disabled in the same release in
https://github.com/payloadcms/payload/pull/11988.

## How to reproduce

### Collection configuration

```ts
const RICH_TEXT_BLOCK_TYPE = 'richTextBlockType'

const RichTextBlock: Block = {
  slug: RICH_TEXT_BLOCK_TYPE,
  interfaceName: 'RichTextBlock',
  fields: [
    {
      name: 'richTextBlockField',
      label: 'Rich Text Field in Block Field',
      type: 'richText',
      editor: lexicalEditor({}),
      required: true,
    },
  ],
}

const MyCollection: CollectionConfig = {
  slug: 'my-collection-slug,
  fields: [
    {
      name: 'arrayField',
      label: 'Array Field',
      type: 'array',
      fields: [
        {
          name: 'blockField',
          type: 'blocks',
          blocks: [RichTextBlock],
          required: true,
        },
      ],
    },
  ]
}

export default MyCollection
```

### Steps

- Press "Add Array Field"
   --> ✅ 1st block with rich text is added
- Press "Add Array Field" a 2nd time

### Result
- 🛑 2nd block is indefinitely in loading state (side-note: the form UI
should preferably explicitly indicate the error).
- 🛑 If saving the document, it is corrupted and will only show a blank
page (also not indicating any error).

Client side:

<img width="1268" alt="Untitled"
src="https://github.com/user-attachments/assets/4b32fdeb-af76-41e2-9181-d2dbd686618a"
/>

API error:

<img width="1272" alt="image"
src="https://github.com/user-attachments/assets/35dc65f7-88ac-4397-b8d4-353bcf6a4bfd"
/>

Client side, when saving and re-opening document (API error of `GET
/admin/collections/${myCollection}/${documentId}` is the same (arguably
the HTTP response status code shouldn't be `200`)):

<img width="1281" alt="image"
src="https://github.com/user-attachments/assets/2e916eb5-6f10-4e82-9b84-1dc41db21d47"
/>

### Result after fix
- `blockType` is sent from the client to the server.
- ✅ 2nd block with rich text is added.
- ✅ Document does not break when saving & re-opening.

<img width="1277" alt="Untitled"
src="https://github.com/user-attachments/assets/84d0c88b-64b2-48c4-864d-610d524ac8fc"
/>

---------

Co-authored-by: Jacob Fletcher <jacobsfletch@gmail.com>

packages/ui/src/forms/fieldSchemasToFormState/addFieldStatePromise.ts

@@ -456,6 +456,10 @@ export const addFieldStatePromise = async (args: AddFieldStatePromiseArgs): Prom
                   value: row.blockType,
                 }
 
+                if (addedByServer) {
+                  state[fieldKey].addedByServer = addedByServer
+                }
+
                 if (includeSchema) {
                   state[fieldKey].fieldSchema = block.fields.find(
                     (blockField) => 'name' in blockField && blockField.name === 'blockType',

test/form-state/int.spec.ts

@@ -228,6 +228,12 @@ describe('Form State', () => {
       collection: postsSlug,
       data: {
         title: 'Test Post',
+        blocks: [
+          {
+            blockType: 'text',
+            text: 'Test block',
+          },
+        ],
       },
     })
 
@@ -248,6 +254,7 @@ describe('Form State', () => {
     })
 
     expect(state.title?.addedByServer).toBe(true)
+    expect(state['blocks.0.blockType']?.addedByServer).toBe(true)
 
     // Ensure that `addedByServer` is removed after being received by the client
     const newState = mergeServerFormState({