fix(eks): looked up vpc causing premature validation errors for private subnets (#33786)

### Issue # (if applicable)

Related to one of the related issue for EKS and VPC in  #22025 .

### Reason for this change

Currently EKS cluster with private endpoint access fails during `cdk synth` operation for a lookedup VPC.
investigating further into lookup implementation, the VPC id is first populated through some dummy values including the one for private subnets :

```
    {
      name: 'Private',
      type: cxapi.VpcSubnetGroupType.PRIVATE,
      subnets: [
        {
          availabilityZone: 'dummy1a',
          subnetId: 'p-12345',
          routeTableId: 'rtb-12345p',
          cidr: '1.2.3.4/5',
        },
```
But there are no dummy values defined for the case of privatesubnetIds :

```
privateSubnetIds: undefined,
```
which results in return of filtering by IDs option as a null object until the values are fully resolved.
Reference code: https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-ec2/lib/vpc.ts#L2705

### Description of changes

using existing field `isPendingLookup` in the SubnetSelection which is being set on the basis of  this.incompleteSubnetDefinition = isIncomplete; where `isIncomplete` is set to false during first pass of cdk synth.
So during first synth operation, validation will be skipped.

### Describe any new or updated permissions being added

NA

### Description of how you validated changes

Added unit test and integration test

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: shikha372