perf(azure): throttle parallel tenant sign-in checks with shared concurrency limiter

[hanhan761]

Summary

Replace unbounded Promise.all fan-outs against the Azure Identity endpoint with a shared ConcurrencyLimiter (concurrency: 5).

For corporate users with many tenants, the original code issued unlimited parallel isSignedIn calls against Microsoft Entra -- the same anti-pattern fixed for estimateDocumentCount in #685.

Issue

Fixes #688

Changes

File	Change
src/plugins/api-shared/azure/tenantSignInLimiter.ts	New -- shared LimitedRunner (concurrency: 5) wrapping createConcurrencyLimiter
src/plugins/api-shared/azure/credentialsManagement/SelectAccountStep.ts	Import limiter, wrap isSignedIn calls
src/plugins/api-shared/azure/subscriptionFiltering/InitializeFilteringStep.ts	Import limiter, wrap isSignedIn calls

Verification

• npm run build -- TypeScript compilation passes
• npm run lint -- ESLint passes
• npm run prettier-fix -- formatting confirmed

[tnaum-ms]

Maintainer note (triage)

This branch was rebased onto main and reduced to the minimal change for #688. It reuses the existing shared createConcurrencyLimiter utility (instead of a bespoke limiter) to cap parallel tenant isSignedIn checks across both wizard steps (SelectAccountStep and InitializeFilteringStep). The unrelated strays that had leaked in via earlier stacked commits (index-count code and the package.json multi-select changes from a sibling PR) were pruned, so the diff is now limited to the limiter and its two call sites.

There was no Copilot review on this PR. The limiter itself is unit-tested; a manual check with a 50+ tenant Azure account is still recommended before merge.