fix: added dns for existing project to access

[AjitPadhi-Microsoft]

Purpose

• This pull request updates the logic for deploying private DNS zones and private endpoints in the infra/modules/ai-foundry/aifoundry.bicep file to better support scenarios where an existing Cognitive Service is used. The main changes ensure that private DNS zones and private endpoints are only created when using an existing service and private networking is enabled.

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Golden Path Validation

• [ ] I have tested the primary workflows (the "golden path") to ensure they function correctly without errors.

Deployment Validation

• [ ] I have validated the deployment process successfully and all services are running as expected with this change.

What to Check

Verify that the following are valid

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Other Information