When caption is applied to embedded media, indicate view mode

The patch is working, thank you!

Previously tagged for tests, which are still needed.

I don't know much about determining what tests are needed or how to write the necessary tests.

I asked ChatGPT to evaluate the patch and suggest tests and this is what I got as a reply:

To ensure the patch works as expected, here are some test cases you should add to cover key areas of functionality. These can be incorporated into Drupal's existing test suite, particularly in a unit test or functional test format.

### **1. Check the Presence of `view-mode--default` Class When `data-view-mode` is Missing**
You need to test the fallback behavior when the `data-view-mode` attribute is absent. This ensures that the class `view-mode--default` is added correctly.

- **Test Case:** Render content with a caption element **without** a `data-view-mode` attribute.
- **Expected Outcome:** The `class` attribute of the caption element should contain `view-mode--default`.

```php
// Test case simulating a caption element without a 'data-view-mode' attribute.
$this->assertContains('view-mode--default', $processed_node->getAttribute('class'));
```

### **2. Check the Correct `view-mode--[mode]` Class When `data-view-mode` is Present**
Test that when a `data-view-mode` attribute is present, the correct `view-mode--[mode]` class is added to the caption element.

- **Test Case:** Render content with a caption element that **has** a `data-view-mode` attribute (e.g., `data-view-mode="teaser"`).
- **Expected Outcome:** The `class` attribute of the caption should include `view-mode--teaser` or the specified view mode.

```php
// Test case for a caption element with a 'data-view-mode' attribute.
$this->assertContains('view-mode--teaser', $processed_node->getAttribute('class'));
```

### **3. Verify Existing Class Attributes are Preserved**
Ensure that existing classes in the `class` attribute are preserved and the new `view-mode--[mode]` class is appended correctly.

- **Test Case:** Render content with a caption element that has existing classes (e.g., `class="some-existing-class"`).
- **Expected Outcome:** The `class` attribute should include both the existing class and the new `view-mode--[mode]` class.

```php
// Test case for an element that has existing classes.
$this->assertContains('some-existing-class', $processed_node->getAttribute('class'));
$this->assertContains('view-mode--default', $processed_node->getAttribute('class'));
```

### **4. Test Nested Elements (e.g., `` Inside `` or `

- **Test Case:** Render content where a captioned element is nested inside an anchor tag (``).
- **Expected Outcome:** The class handling should work properly, ensuring the correct class is added to the appropriate node.

```php
// Test case for a caption element inside an tag.
$this->assertContains('view-mode--default', $parent_node->getAttribute('class'));
```

### **5. Ensure No Duplicates in the Class Attribute**
Test that no duplicate `view-mode--[mode]` classes are added, even if the `data-view-mode` is processed multiple times.

- **Test Case:** Process a node multiple times with the same `data-view-mode`.
- **Expected Outcome:** The `view-mode--[mode]` class should only appear once.

```php
// Test case to check for duplicate class names.
$this->assertSame(1, substr_count($processed_node->getAttribute('class'), 'view-mode--default'));
```

### **6. Test Integration with Other Filters (e.g., Filter Align)**
Since the comment mentions collaboration with other filters like `filter_align`, ensure that the patch doesn’t break any existing functionality where alignment filters might also manipulate the class attributes.

- **Test Case:** Apply the `filter_align` filter to the captioned element in conjunction with the `data-view-mode` modification.
- **Expected Outcome:** The class attributes for both the view mode and alignment are applied correctly.

```php
// Test case where another filter like 'filter_align' collaborates with this patch.
$this->assertContains('align-left', $processed_node->getAttribute('class'));
$this->assertContains('view-mode--default', $processed_node->getAttribute('class'));
```

### **7. Test with Multiple View Modes**
Ensure that the patch works with various view modes (e.g., `teaser`, `full`, `custom`).

- **Test Case:** Render content with different `data-view-mode` values.
- **Expected Outcome:** The class should change dynamically to match the view mode provided.

```php
// Test case for multiple view modes like 'teaser', 'full', etc.
$this->assertContains('view-mode--full', $processed_node->getAttribute('class'));
$this->assertContains('view-mode--custom', $processed_node->getAttribute('class'));
```

### Additional Considerations
- **Sanitization:** Ensure that any `data-view-mode` values are sanitized or validated to avoid injecting invalid class names.
- **Edge Cases:** Test behavior when the `data-view-mode` contains unexpected or malformed values, such as special characters.

### Summary
These test cases will ensure that your patch is robust and works as expected across a variety of scenarios, from simple view modes to more complex interactions with nested elements and other filters. You can use Drupal's `KernelTestBase` or `BrowserTestBase` for functional tests, depending on how deeply integrated the changes are with the rendered content and front-end behavior.

This patch worked perfectly! (on 10.3.5)

Thank you.

How do we get this in to core?