MOD-15579 Lazy initialization of the shared SVS thread pool

[meiravgri]

Make initialization of the shared SVS thread pool lazy: OS worker threads are spawned only when the first SVS index is created, not when VecSim_UpdateThreadPoolSize is called. This eliminates the cost of pre-allocated SVS workers in deployments that configure RediSearch workers but never create an SVS index (today, every Redis with WORKERS=N configured spawns N-1 SVS worker threads at module init regardless of whether SVS is ever used).

The change extends the existing deferred-resize protocol to cover a second "safe point". VecSimSVSThreadPoolImpl::resize() now defers application in two cases:

• No SVS index attached yet → recorded; applied on first onIndexAttached() (lazy thread spawn).
• Shrink while jobs are in flight (pending_jobs_ > 0) → recorded; applied when endScheduledJob() drops the counter to 0 (existing behaviour).
• Otherwise → applied immediately via resize_locked().

The two deferral cases share deferred_size_ and never overlap (no jobs can be in flight before the first index attaches). One new gate field, has_attached_index_, distinguishes the two states. Set on the first onIndexAttached() call from the per-index VecSimSVSThreadPool ctor.

A test-only resetForTest() hook (under BUILD_TESTS) restores the singleton to a clean baseline by releasing the slots vector capacity and clearing the new gate; intended for unit tests that need to observe lazy semantics in a process where prior tests may have already attached indexes.

VecSim_UpdateThreadPoolSize continues to set the write mode (InPlace/Async) eagerly — only the SVS pool sizing is now deferred. The C API surface is unchanged.

RediSearch integration impact: none required — VecSim_UpdateThreadPoolSize(N) is still the single entry point for both module init and CONFIG SET search-workers M. Pre-index calls now just record the size; post-index calls behave exactly as before.

Main objects this PR modified

1. VecSimSVSThreadPoolImpl::resize() — now lazy until first index attaches.
2. VecSimSVSThreadPoolImpl::onIndexAttached() — new entry point called from per-index VecSimSVSThreadPool ctor; flips has_attached_index_ and applies any deferred size.
3. VecSimSVSThreadPoolImpl::has_attached_index_ — new member field gating spawn vs. defer.
4. VecSimSVSThreadPoolImpl::resetForTest() — new BUILD_TESTS-only hook.
5. VecSim_UpdateThreadPoolSize() — comment updated; behaviour now lazy via the new resize() semantics.

Tests

• SVSTest.ThreadPoolLazyInit (new) — verifies VecSim_UpdateThreadPoolSize does not allocate worker threads before any SVS index exists, and that the first SVS index creation triggers the deferred spawn at the previously requested size.
• SVSThreadPoolTest.UpdateThreadPoolSizeModeTransitions — rewritten to validate the lazy → eager transition (size stays at 1 until an index attaches; subsequent transitions are immediate).
• SVSThreadPoolTest fixture — SetUp() calls onIndexAttached() so the existing pool-isolation tests retain eager resize() semantics.
• SVSTest.NumThreadsParamIgnored — calls onIndexAttached() upfront for the same reason.
• SVSTest.debugInfoGlobalMemoryEqualsSharedSVSThreadPoolMemory — ASSERT_GT on global memory moved to after CreateNewIndex (the lazy spawn fires there).

Mark if applicable

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

---

Note

Medium Risk
Changes shared SVS thread-pool resizing semantics and introduces a new lazy-init/deferral state, which can affect concurrency and job scheduling behavior if edge cases were missed. Unit tests were updated/added, but threading/resource-management changes still carry moderate risk.

Overview
Defers creation of SVS shared pool worker threads until the first SVS index attaches: VecSimSVSThreadPoolImpl::resize() now records requested sizes pre-index and applies them in onIndexAttached(), while keeping the existing defer shrink while jobs are in flight behavior.

Adds a BUILD_TESTS-only resetForTest() to clear singleton state, updates VecSim_UpdateThreadPoolSize() documentation to reflect lazy spawning, and adjusts/extends unit tests to validate the lazy→eager transition and global-memory reporting (including a new ThreadPoolLazyInit test).

^{Reviewed by Cursor Bugbot for commit 7bc3e24. 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

---

^{Security scan by Jit}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.93%. Comparing base (31c0e0a) to head (7bc3e24).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           meiravg_svs_thpool_alloactor     #969      +/-   ##
================================================================
- Coverage                         96.97%   96.93%   -0.04%     
================================================================
  Files                               130      130              
  Lines                              7829     7801      -28     
================================================================
- Hits                               7592     7562      -30     
- Misses                              237      239       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.