fix: SUI-1973 - support existing storage resources

[IgiCodes]

Summary

Adds safer support for the blob-event-processor stack when using a pre-existing storage account, while reducing expected what-if noise from Azure-managed defaults and DfE policy tags.

Changes

• Treat storage account blobServices/default and queueServices/default as existing service handles, so the stack only manages the SUI-owned containers, queues, private endpoints, DNS, and Event Grid wiring.
• Preserve default blob container properties explicitly to avoid repeated container drift in what-if output.
• Add optional tag controls for policy-shaped environments:
  • tagEnvironmentName lets deployments align the Environment tag with policy expectations.
  • additionalTags / AZURE_ADDITIONAL_TAGS lets DfE-specific tags such as Service Offering be supplied from repo variables instead of being hard-coded into client infrastructure.
• Pass the shared stack tags into Key Vault without replacing its required Aspire tag.
• Document existing-storage behaviour and the optional tag configuration for GitHub Actions and direct deployments.

Validation

• Built infra/stacks/blob-event-processor/main.bicep with az bicep build.
• Built infra/stacks/blob-event-processor/subscription.bicep with az bicep build.
• Ran create-mode what-if from the branch and confirmed the storage account, containers, queues, private endpoints, and Event Grid resources are no-change.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Package	Line Rate	Branch Rate	Health
suiw	100%	100%	✔
SUI.Client.Core	94%	91%	✔
Matching	74%	49%	➖
External	45%	32%	❌
Shared	92%	84%	✔
AppHost	100%	100%	✔
SUI.Client.StorageProcessJob	97%	80%	✔
SUI.DBS.Response.Logger.Core	94%	76%	✔
Summary	80% (3816 / 4773)	63% (1001 / 1600)	✔