aws-elasticloadbalancingv2: Missing validation for protocol in the ApplicationTargetGroup construct

### Describe the bug

When creating an ApplicationTargetGroup in AWS CDK without specifying a protocol, the error is not caught at synthesis time. Instead, it only appears during deployment as a runtime error from the Elastic Load Balancing service:
```
Resource handler returned message: "A protocol must be specified (Service: ElasticLoadBalancingV2, Status Code: 400) (SDK Attempt Count: 1)"
```

This results in wasted time and failed deployments, even though the required protocol is a simple, well-defined property that could easily be validated during synthesis.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

The ApplicationTargetGroup construct should validate that the protocol property is specified during synthesis. If missing, CDK should raise a synthesis-time error, preventing failed deployments and improving developer experience.

### Current Behavior

Currently, CDK allows creating a target group without a protocol. The error is only detected during deployment. Example of failing code:
```
this.targetGroup = new ApplicationTargetGroup(this, 'TargetGroup', {
vpc: this.vpc
});
```
The code can be fixed by specifying the protocol:
```
this.targetGroup = new ApplicationTargetGroup(this, 'TargetGroup', {
vpc: this.vpc,
protocol: ApplicationProtocol.HTTPS,
});
```

### Reproduction Steps

N/A

### Possible Solution

Since the protocol property is mandatory for ApplicationTargetGroup, adding validation at synthesis time would prevent runtime errors and save developers from failed deployments.

### Additional Information/Context

_No response_

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

2.207.0

### AWS CDK CLI version

2.1022.0

### Node.js Version

18.20.2

### OS

MacOS

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start at the ApplicationTargetGroup construct and inspect how its protocol property is currently handled during synthesis. Add coverage for a missing protocol and confirm that synthesis reports the error instead of allowing the deployment-time Elastic Load Balancing failure.

Written by the indexing model from the issue text.