Incorrect validation error for crowd-classifier-multi-select: "src" attribute should not be required

**Labels:** `bug`, `documentation`

**Description:**

## Bug Description

The SageMaker Ground Truth template validator incorrectly reports a validation error for `` claiming that the `src` attribute is required, when in fact this element does not use a `src` attribute.

**Error message:**
```
Potential errors
We found some potential errors with your usage of Crowd HTML Elements.

(Element type CROWD-CLASSIFIER-MULTI-SELECT): attributes should have required property 'src'
```

## Expected Behavior

`` should **not** require a `src` attribute. According to the [[official AWS documentation](https://docs.aws.amazon.com/sagemaker/latest/dg/sms-ui-template-crowd-classifier-multi-select.html)](https://docs.aws.amazon.com/sagemaker/latest/dg/sms-ui-template-crowd-classifier-multi-select.html), this element uses a `` child element instead of a `src` attribute.

The element has the following **required** attributes:
- `name`
- `categories`
- `header`

And uses `` as a child element to contain the content to be classified.

## Actual Behavior

The validator throws a false positive error claiming `src` is required, even when the template is correctly structured with ``.

## Template Example That Triggers the Error

```html

{{ task.input.taskObject }}

Positive sentiment include: joy, excitement, delight

Negative sentiment include: anger, sarcasm, anxiety

Neutral: neither positive or negative, such as stating a fact

N/A: when the text cannot be understood

When the sentiment is mixed, such as both joy and sadness, choose both labels.

Choose all categories that are expressed by the text.

```

This template is **correct** according to the documentation but triggers the validation error.

## Impact

This validation error is confusing for users creating custom templates, as it:
1. Suggests the template is incorrect when it's actually valid
2. May cause users to incorrectly add a `src` attribute that doesn't belong
3. Creates uncertainty about whether the template will work in production

## Additional Context

- The single-select version `` does **not** trigger this validation error when using the same structure
- The documentation clearly shows this element should use ``, not `src`
- This appears to be a bug in the validation logic, possibly conflating `` with `` (which **does** require a `src` attribute)

## Reproduction Steps

1. Go to SageMaker Ground Truth console
2. Create a custom labeling job template
3. Use the template code provided above
4. Observe the validation error appears despite the template being correct per documentation

## Suggested Fix

Update the validation schema for `` to:
- Remove `src` from the required attributes list
- Ensure it validates for `` child element instead

## Environment

- Service: Amazon SageMaker Ground Truth
- Console Region: `us-east-1`
- Date observed: January 2026

## References

- [[Official Documentation: crowd-classifier-multi-select](https://docs.aws.amazon.com/sagemaker/latest/dg/sms-ui-template-crowd-classifier-multi-select.html)](https://docs.aws.amazon.com/sagemaker/latest/dg/sms-ui-template-crowd-classifier-multi-select.html)
- [[Official Documentation: crowd-image-classifier-multi-select](https://docs.aws.amazon.com/sagemaker/latest/dg/sms-ui-template-crowd-image-classifier-multi-select.html)](https://docs.aws.amazon.com/sagemaker/latest/dg/sms-ui-template-crowd-image-classifier-multi-select.html) (for comparison - this one DOES use `src`)