documentation generator does not gracefully catch unclosed code blocks

### Describe the bug

See https://github.com/aws/aws-cdk/issues/20623

Missing code block causes CDK documentation to be corrupted. In addition to fixing the instance, we should try to throw an error if this happens.

### Expected Behavior

An error thrown, or some way to make sure the documentation that shows up is not corrupted

### Current Behavior

Weird documentation shows up.

### Reproduction Steps

See https://github.com/aws/aws-cdk/issues/20623

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### SDK version used

CDK

### Environment details (OS name and version, etc.)

MacOs

Start with the reproduction details in aws/aws-cdk#20623 and trace the jsii documentation generator's handling of code blocks. Compare the generated output for an unclosed block with the expected behavior, then add coverage and verify that the generator reports the problem or prevents corrupted documentation.

Written by the indexing model from the issue text.