Refactor SetupRunner to Correctly Handle Managed Identity in Azure vs External Hosting Scenarios

[mengyimicro]

This PR refactors A365SetupRunner to correctly separate the behavior of Azure-hosted deployments, blueprint-only mode, and external (non-Azure) hosting.

Previously, both blueprint-only and external hosting paths flowed through the same logic, causing the managed identity (MSI) logic to be reused incorrectly.

This change introduces clear separation and ensures that Managed Identity, Federated Identity Credential (FIC), and Azure infra provisioning occur only when appropriate.

1. NeedDeployment = true (Azure hosting)

• Create/ensure web app.
• Assign system-assigned MSI.
• Use its principalId to:
  ** Store managedIdentityPrincipalId in a365.generated.config.json.
  ** Create Federated Identity Credential (FIC) in CreateAgentBlueprintAsync.

2. Blueprint-only (--blueprint = true)

• No new web app or MSI.
• If an existing managedIdentityPrincipalId is present from a previous run, re-use it for FIC (optional but nice).
• Otherwise, just create blueprint without FIC.

3. External hosting (needDeployment = false, no --blueprint)

• No web app, no MSI, no FIC.
• Blueprint uses client secret only (no managed identity).

[Copilot]

Pull request overview

This PR refactors the A365SetupRunner to correctly handle three distinct hosting scenarios: Azure-hosted deployments (with managed identity), blueprint-only mode (reusing existing managed identity), and external hosting (no managed identity). The changes introduce explicit separation of these modes through the externalHosting and useManagedIdentity variables, ensuring that Managed Identity and Federated Identity Credential creation only occur when appropriate.

Key Changes:

• Introduced externalHosting variable to distinguish external hosting from blueprint-only mode
• Added useManagedIdentity parameter to CreateAgentBlueprintAsync to control FIC creation
• Updated configuration loading logic to handle external hosting differently from blueprint-only mode

[skg-marimuthu]