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

[github-actions]

Description

Backport of #931 to 8.2.

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Note

Low Risk
Changes only affect debug/status reporting and a flow test assertion; core indexing/search logic is untouched, so functional risk is low aside from potentially different background-indexing flags in diagnostics.

Overview
Prevents TieredSVSIndex::debugInfo() from blocking while SVS background training holds updateJobMutex by switching to try_lock; when the mutex is busy, it reports background indexing as active instead of waiting.

Updates the test_svs_tiered flow test to stop asserting that the flat-buffer size monotonically decreases during background indexing (removing a timing/flakiness-sensitive check).

^{Reviewed by Cursor Bugbot for commit 2f97044. 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 96.98%. Comparing base (09adad1) to head (2f97044).

Additional details and impacted files

@@            Coverage Diff             @@
##              8.2     #932      +/-   ##
==========================================
- Coverage   97.01%   96.98%   -0.03%     
==========================================
  Files         126      126              
  Lines        7364     7366       +2     
==========================================
  Hits         7144     7144              
- Misses        220      222       +2     

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