(ec2): add p6-b200, p6-b300, and g7e instances to EC2 instance types

### Describe the feature

Add the p6-b200, p6-b300, and g7e instance classes to ec2 for creating instances with the new classes via cdk

### Use Case

I want to create ec2 instances with the new instance classes: p6-b200, p6-b300, and g7e instance classes via cdk

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

v2.254.0

### AWS CDK CLI version

2.1106.0

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

macOS 26.4.1 (25E253)

Start by locating the AWS CDK EC2 instance-type definitions and nearby tests for supported instance classes. Add coverage for p6-b200, p6-b300, and g7e, then run the relevant EC2 tests to confirm each class can be used when creating an instance.

Written by the indexing model from the issue text.