feat: Enable system assigned managed identity for existing Foundry project and add reuse documentation

[Prasanjeet-Microsoft]

Purpose

• This PR enables the assignment of System Assigned Managed Identity (SAMI) to an existing Azure AI Foundry project when it is reused in deployments. This ensures that role-based access can be granted even when the AI project already exists.
• Additionally, it introduces a README section that documents how to reuse an existing Azure AI Foundry project. This includes step-by-step instructions for locating the project in the Azure Portal, extracting its Resource ID, and configuring it via azd env set.

Does this introduce a breaking change?

• Yes
• 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:

• The system-assigned managed identity is successfully enabled for reused (existing) AI Foundry projects.

• The identity appears in the Azure Portal under the Identity section of the AI project.

• No new project is created when reusing an existing one.

• Role assignments dependent on the system-assigned identity work correctly.

• Output values (name, resource ID, API endpoint) are accurate for both new and existing projects.

• The behavior for creating a new AI project remains unchanged and unaffected.

• The reuse guide in the README:

  • Follows the format of existing documentation (e.g., Log Analytics workspace reuse)
  • Accurately describes how to navigate Azure Portal and locate the AI Foundry project's Resource ID
  • Uses the correct azd command to configure the environment variable
  • Aligns with the expected behavior in single-project Foundry services