MOD-15579 Lazy initialization of the shared SVS thread pool

[dor-forer]

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

Supersedes #969 — ownership transferred to @dor-forer.

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Note

Medium Risk
Changes when OS threads and pool memory are created (module init vs first SVS index), but defers to existing deferred-shrink safety and is covered by new/updated unit tests; C API entry point unchanged.

Overview
Defers spawning SVS worker OS threads until the first SVS index is created. VecSim_UpdateThreadPoolSize still sets write mode immediately, but pool sizing is only recorded in deferred_size_ until onIndexAttached() runs from the per-index VecSimSVSThreadPool constructor.

resize() now gates on has_attached_index_: before any index attaches it stores the target size; after attach it uses the existing resize_locked() path (including shrink-while-jobs-in-flight deferral via pending_jobs_). The two deferral cases share deferred_size_ and are documented as non-overlapping.

Adds BUILD_TESTS-only resetForTest() so unit tests can observe lazy semantics without a process-wide singleton teardown. Tests cover lazy init, lazy→eager UpdateThreadPoolSize transitions, and fixture hooks that call onIndexAttached() where tests resize the pool without building a real index.

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.97%. Comparing base (a923fa7) to head (09d0e83).

Additional details and impacted files

@@                               Coverage Diff                               @@
##           dor-forer-MOD-15578-track-svs-thpool-memory     #971      +/-   ##
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- Coverage                                        97.08%   96.97%   -0.12%     
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  Files                                              141      130      -11     
  Lines                                             8144     7843     -301     
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- Hits                                              7907     7606     -301     
  Misses                                             237      237              

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