[8.2] MOD-14618 Fix race condition when calling indexLabelCount on SVS without locks

[github-actions]

Description

Backport of #919 to 8.2.

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Note

Medium Risk
Touches tiered-index concurrency by adding locking around sub-index debug-info access; low functional impact but mistakes could cause contention or deadlocks under background indexing.

Overview
Prevents races when collecting tiered index debug output by acquiring shared_locks on flatIndexGuard and mainIndexGuard before calling frontendIndex->debugInfoIterator() and backendIndex->debugInfoIterator().

Locks are scoped per sub-index (instead of held across both calls) to keep the critical sections small while still making debug-info generation safe during background indexing.

^{Reviewed by Cursor Bugbot for commit 6e89436. Bugbot is set up for automated code reviews on this repo. Configure here.}

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.01%. Comparing base (09adad1) to head (6e89436).

Additional details and impacted files

@@           Coverage Diff           @@
##              8.2     #930   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files         126      126           
  Lines        7364     7366    +2     
=======================================
+ Hits         7144     7146    +2     
  Misses        220      220           

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