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. It is set on the first onIndexAttached() call from the per-index VecSimSVSThreadPool ctor.

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 the first index attaches.
2. VecSimSVSThreadPoolImpl::onIndexAttached() — new entry point called from the 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 that restores the singleton to a clean baseline. Asserts pending_jobs_ == 0 so misuse fails loudly instead of corrupting the deferred-protocol bookkeeping.
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. Post-VecSimIndex_New checks use ASSERT_* so a regression in the lazy-spawn contract halts the test on the first failure.
• SVSThreadPoolTest.UpdateThreadPoolSizeModeTransitions — validates the lazy → eager transition (pool size stays at 1 until an index attaches; subsequent transitions apply immediately).
• SVSThreadPoolTest fixture — SetUp() calls onIndexAttached() so the pool-isolation tests retain eager resize() semantics.
• SVSTest.NumThreadsParamIgnored — calls onIndexAttached() up front (resizes before constructing an index) and restores the singleton via resetForTest() so it does not leak has_attached_index_ to later tests.
• SVSTest.sharedMemoryTracksThreadPoolResize — calls onIndexAttached() up front so the resize-without-index path applies eagerly and the grow/shrink memory deltas are observable.

Mark if applicable

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

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Supersedes #969 — ownership transferred to @dor-forer. Builds on #972 (MOD-15578), now merged to main.

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Note

Medium Risk
Changes shared thread-pool lifecycle and when OS threads are created, but the public C API is unchanged and deferred-shrink while jobs are in flight is preserved; risk is mainly regressions in worker count or timing around module init vs first SVS index.

Overview
Lazy SVS thread pool sizing defers spawning OS worker threads until the first SVS index is created. VecSim_UpdateThreadPoolSize still switches write mode (InPlace / Async) immediately; only pool sizing is deferred.

VecSimSVSThreadPoolImpl::resize() now records the requested size in deferred_size_ when no index has attached (has_attached_index_ is false) instead of calling resize_locked() right away. The first per-index VecSimSVSThreadPool construction calls onIndexAttached(), which sets the flag and applies any deferred size—this is where workers are created on the lazy path. After attach, resize behaves as before (immediate grow/shrink, with shrink still deferred while pending_jobs_ > 0).

A BUILD_TESTS-only resetForTest() clears slots, deferred_size_, and has_attached_index_ so unit tests get a clean singleton. Tests add ThreadPoolLazyInit, call onIndexAttached() where tests resize without a real index, and extend UpdateThreadPoolSizeModeTransitions for the lazy → eager transition.

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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 97.09%. Comparing base (ca937db) to head (6f073fb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #971      +/-   ##
==========================================
- Coverage   97.11%   97.09%   -0.02%     
==========================================
  Files         141      141              
  Lines        8144     8160      +16     
==========================================
+ Hits         7909     7923      +14     
- Misses        235      237       +2     

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