perf: throttle parallel Azure tenant sign-in checks

[tnaum-ms]

Context

Surfaced during the review for #685.

Location

Two near-identical fan-outs against the Azure Identity endpoint:

• src/plugins/api-shared/azure/credentialsManagement/SelectAccountStep.ts around line 146
• src/plugins/api-shared/azure/subscriptionFiltering/InitializeFilteringStep.ts around line 67

Both do:

await Promise.all(tenants.map(async (tenant) => isSignedIn(tenant.tenantId, tenant.account)));

The shape of the problem

For most users (1-10 tenants), this is fine. For corporate users with many tenants, this is an unbounded parallel burst against Microsoft Entra. It is not currently a known cause of failures, but it is the same anti-pattern that bit us in #685 with estimateDocumentCount.

Proposal

Apply the shared ConcurrencyLimiter from src/utils/concurrencyLimiter.ts (added in #685). Suggested starting point: concurrency: 5, no inter-batch delay (this is a foreground UX path, latency matters). Both call sites should share a single limiter so the total parallelism is bounded across the wizard.

Acceptance criteria

• Both call sites routed through a shared limiter.
• Behavior unchanged for users with a small tenant count.
• No regression in the account-selection / subscription-filtering wizards.

References

• PR perf(tree): throttle background document-count fetches #685: introduces the limiter primitive.