[8.4] MOD-14826 Use try_lock in debugInfo() to avoid blocking main thread during SVS training

[github-actions]

Description

Backport of #931 to 8.4.

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Note

Low Risk
Low risk: changes only affect debug/info reporting and a test assertion, with no impact on indexing/query logic beyond how background-indexing status is reported during long training runs.

Overview
Prevents TieredSVSIndex::debugInfo() from blocking on updateJobMutex during long-running SVS training by switching to a non-blocking try_lock and treating a held mutex as training in progress (reports indexUpdateScheduled=true).

Updates the tiered SVS flow test to stop asserting that the flat-buffer size monotonically decreases during background indexing, and instead asserts the buffer is empty once indexing completes.

^{Reviewed by Cursor Bugbot for commit 89bcd90. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.06%. Comparing base (5e28cb6) to head (89bcd90).
⚠️ Report is 1 commits behind head on 8.4.

Additional details and impacted files

@@           Coverage Diff           @@
##              8.4     #933   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files         126      126           
  Lines        7463     7465    +2     
=======================================
+ Hits         7244     7246    +2     
  Misses        219      219           

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