[WIP] Refactor BlobClient into a StorageBlobService for per-call subscription

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Refactor BlobClient into a StorageBlobService following per-call subscription pattern</issue_title>
<issue_description>## Summary

The current BlobClient in cli/azd/pkg/azsdk/storage/ is architecturally unique compared to other Azure service clients in the codebase. It binds the subscription/tenant at construction time via AccountConfig, whereas all other services (KeyVault, ContainerApps, ContainerRegistry, AppService, etc.) accept subscriptionId per-call and store only a SubscriptionCredentialProvider.

Current pattern (BlobClient — exception)

// Constructor locks in subscription via AccountConfig
func NewBlobSdkClient(credentialProvider, accountConfig *AccountConfig, ...) (*azblob.Client, error)

// Methods have no subscription parameter
type BlobClient interface {
    Download(ctx context.Context, blobPath string) (io.ReadCloser, error)
    Upload(ctx context.Context, blobPath string, reader io.Reader) error
    // ...
}

Standard pattern (KeyVault, ContainerApps, etc.)

// Constructor takes credential provider only
func NewKeyVaultService(credentialProvider, ...) KeyVaultService

// Methods accept subscriptionId per-call
GetKeyVault(ctx, subscriptionId, resourceGroup, vaultName) (*KeyVault, error)

Interestingly, FileShareService in the same storage package already follows the standard per-call pattern.

Proposal

Refactor the blob storage abstraction into a StorageBlobService that:

1. Follows the per-call subscriptionId pattern used by other Azure services
2. Stores a SubscriptionCredentialProvider rather than pre-resolved credentials
3. Can be consumed by the remote state feature (and any future blob storage consumers) without being locked to a single subscription at construction time

This is a code & design improvement — the current implementation works correctly for its single use case (remote state), but consolidating the pattern improves consistency and future extensibility.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Refactor BlobClient into a StorageBlobService following per-call subscription pattern #7073

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