fix(providers/azure): use SDK enum constant for appliedScopeType in purchase bodies

[cristim]

Context

During adversarial review of PR LeanerCloud/cloud-commitments-cli#1208, all Azure service clients were found to use `"Shared"` as a raw string literal for the `appliedScopeType` field in reservation purchase request bodies.

The Azure SDK defines a typed enum for this:

```go
armreservations.AppliedScopeTypeShared // = "Shared"
armreservations.AppliedScopeTypeSingle // = "Single"
```

Files affected

All service clients that build a purchase request body:

• `providers/azure/services/cache/client.go`
• `providers/azure/services/compute/client.go`
• `providers/azure/services/cosmosdb/client.go`
• `providers/azure/services/database/client.go`
• `providers/azure/services/managedredis/client.go`
• `providers/azure/services/search/client.go`
• `providers/azure/services/synapse/client.go`

Fix

Replace `"appliedScopeType": "Shared"` with `"appliedScopeType": string(armreservations.AppliedScopeTypeShared)` in each client's `PurchaseCommitment` method (7 occurrences, one per service).

Note

This is a code-quality / forward-safety improvement only. The current literal `"Shared"` is the correct value and matches the SDK constant; there is no live bug here. The risk is that a typo in a future edit would not be caught at compile time.

This is a follow-up to PR LeanerCloud/cloud-commitments-cli#1208 which fixed the higher-priority `reservedResourceType` enum correctness bugs.