Fix race condition in VecSimTieredIndex::debugInfoIterator during background indexing

[JoanFM]

Describe the changes in the pull request

Fixes a race condition that causes a crash when debugInfoIterator() is called on a tiered SVS index while background indexing is running.

Root Cause Analysis

The crash occurs because VecSimTieredIndex::debugInfoIterator() calls backendIndex->debugInfoIterator() without holding any locks. During async background indexing, the backend SVS index's internal state (impl_) can be modified by the background thread while the main thread is accessing it through debugInfo() → indexLabelCount() → impl_->size().

Stack trace from the crash:

SVSIndex::indexLabelCount
  ← SVSIndex::debugInfo
  ← SVSIndex::debugInfoIterator  
  ← VecSimTieredIndex::debugInfoIterator (NO LOCK HELD)
  ← TieredSVSIndex::debugInfoIterator
  ← VecsimInfo

The interleaved log messages showed that background indexing was running (rounds 2-12 of 40) when VecsimInfo was called, confirming the race.

Fix

Acquire shared locks (flatIndexGuard and mainIndexGuard) before calling frontendIndex->debugInfoIterator() and backendIndex->debugInfoIterator(). This follows the same lock ordering pattern used elsewhere in the codebase (e.g., in debugInfo() and indexLabelCount()).

Main objects this PR modified

1. VecSimTieredIndex::debugInfoIterator() in vec_sim_tiered_index.h

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

---

Note

Medium Risk
Touches concurrency/locking around tiered index introspection; while scoped to debug info, incorrect locking could still introduce contention or deadlocks under load.

Overview
Fixes a crash/race in VecSimTieredIndex::debugInfoIterator() by acquiring shared_locks on flatIndexGuard and mainIndexGuard when generating the frontend/backend sub-index debug iterators.

Locks are taken per sub-index (and released immediately after) to keep the iterator construction thread-safe during background indexing while minimizing lock contention.

^{Written by Cursor Bugbot for commit 409ffff. This will update automatically on new commits. Configure here.}

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

---

^{Security scan by Jit}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.98%. Comparing base (3dd2dc2) to head (409ffff).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #919   +/-   ##
=======================================
  Coverage   96.98%   96.98%           
=======================================
  Files         129      129           
  Lines        7567     7569    +2     
=======================================
+ Hits         7339     7341    +2     
  Misses        228      228           

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