feat: improve interactive configuration UX and prevent empty subscription values for Azure/avm-ptn-alz

[Richard Cheney (richeney)]

This change improves the interactive configuration experience for accelerator setup and avoids downstream bootstrap issues caused by empty subscription values.

What changed

• Improved default-selection behavior in the shared menu helper so it respects an existing value before falling back to index ordering.
• Clarified manual-entry handling and default option behavior for interactive prompts.
• Added more intuitive subscription selection defaults for workload-style keys such as management, connectivity, identity, and security.
• Kept optional values truly optional so users can intentionally leave them blank without being forced into a selection.
• Prevented empty subscription IDs from being passed through to downstream bootstrap and AVM configuration calls, avoiding Terraform/bootstrap failures.

Why

The interactive configuration flow was occasionally forcing unnecessary choices or carrying empty subscription values into later steps. This update makes the prompts more user-friendly while protecting configuration generation from invalid or blank subscription data.

[Richard Cheney (richeney)]

The key fix is for the subscription_ids section of the inputs.yaml. Example:

## Decision 7: Platform Subscriptions
subscription_ids:
  management: "50ace030-4f1f-4792-8efb-f0ccf40f4035"
  identity: ""
  connectivity: "72dab7e3-809b-4b71-b738-847d0e3cf601"
  security: ""

This causes downstream issues. The avm-ptn-alz Terraform module will error on duplicate or non-GUID values in the subscription_placement object.

This PR updates the interactive experience in this module to ensure that any null or empty subscriptions are not created in the input.yaml. E.g.:

## Decision 7: Platform Subscriptions
subscription_ids:
  management: "50ace030-4f1f-4792-8efb-f0ccf40f4035"
  connectivity: "72dab7e3-809b-4b71-b738-847d0e3cf601"

Fixes Azure/Azure-Landing-Zones#4239.

[Richard Cheney (richeney)]

Additional user experience enhancements:

• subscription_ids are shown in required order, i.e. management, connectivity, identity, security. Required subscription are always shown first, optional subscriptions last.
• The required_subscription_ids default of ["management","connectivity"] is set to ["management"] for the Management Only scenario.
• The defaults are now shown consistently. Previously some defaults were in green, some in cyan, some without highlighting. Now they are always shown in green.
• More intelligent defaults.
  • The bootstrap_subscription_id now defaults to the subscription from Get-AzContext, rather than just first in the list.
  • The bootstrap region now defaults to AZURE_DEFAULTS_REGION if that environment variable is set.
  • Subscription name matching for the platform subscriptions. Full exact match (e.g. "Management", or word match (e.g. "Platform - Management") if it is the only match. User can always select a different one. Also the platform subscription in the prompt is shown in yellow to make it stand out.
• Fixed some of the prompts, e.g. the GitHub org prompt mentioned a non-existent default.