Add rule E3064: duplicate Interface VPC Endpoint with PrivateDnsEnabled

[kddejong]

Summary

Add rule E3064 to detect when multiple AWS::EC2::VPCEndpoint resources in the same template have the same VpcId, ServiceName, VpcEndpointType: Interface, and PrivateDnsEnabled: true. AWS rejects the second endpoint at deploy time with a conflicting DNS domain error, causing a stack rollback.

What was tested

• Deployed test stacks to AWS (us-east-1) confirming the EC2 API rejects duplicate Interface endpoints with Private DNS enabled in the same VPC — for any service, not just execute-api
• Verified edge cases: different VPCs (allowed), different services (allowed), mixed PrivateDnsEnabled (allowed), Gateway type duplicates (separate failure mode)

Test cases (12)

• First endpoint in VPC (valid)
• Same service, different VPC (valid)
• Different service, same VPC (valid)
• PrivateDnsEnabled false (valid)
• Gateway type duplicate (valid — different failure mode)
• Mutually exclusive conditions via Fn::If on PrivateDnsEnabled (valid)
• Fn::If on VpcId matching saved in same condition (error)
• Fn::If on PrivateDnsEnabled resolving to false (valid)
• Same conditions on both endpoints (error)
• Plain duplicate, no conditions (error)
• Fn::If on ServiceName matching saved in same condition (error)
• Hardcoded VPC ID duplicate (error)

Fixes #4352

[codecov]

Codecov Report

❌ Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.51%. Comparing base (a1b97ea) to head (03de5f2).

Files with missing lines	Patch %	Lines
.../resources/ectwo/VpcEndpointPrivateDnsDuplicate.py	98.43%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4492      +/-   ##
==========================================
+ Coverage   93.49%   93.51%   +0.02%     
==========================================
  Files         460      461       +1     
  Lines       15037    15101      +64     
  Branches     2907     2924      +17     
==========================================
+ Hits        14059    14122      +63     
  Misses        599      599              
- Partials      379      380       +1     

Flag	Coverage Δ
unittests	93.51% <98.43%> (+0.02%)	⬆️

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