SERVER-130777: fix(index-builds): persist side-write multikey state for resumable index builds

[Abuhaithem]

Fixes: SERVER-130777

Fixes a resumable index build defect that can commit an index which contains multikey entries
but is not flagged multikey, causing the query planner to silently return incorrect results.

A hybrid index build tracks the multikey information generated by concurrent (side) writes only in
memory, in IndexBuildInterceptor::_multikeyPaths; the side writes table rows store bare index
keys. The resume state persisted on clean shutdown captures multikey state only from the bulk
builder — i.e. only what the collection scan observed:

indexStateInfo.setIsMultikey(index.bulk->isMultikey());
for (const auto& multikeyPath : index.bulk->getMultikeyPaths()) { ... }

After a restart the interceptor is reconstructed empty, and re-draining the side writes table
cannot recover the information — applyIndexBuildSideWrite() applies one bare key per row with an
empty MultikeyPaths{}, so shouldMarkIndexAsMultikey(1, {}, {}) never fires. At commit,
setMultikey() is never called for those writes.

Repro shape: scalar-only collection → build passes the collection scan → client inserts
{a: [3, 4]} (captured only by the interceptor) → clean shutdown → resume → commit. The committed
index reports isMultiKey: false, find({a: {$gte: 4, $lte: 3}}) intersects bounds to an empty
interval and misses the array document, and validate({full: true}) fails.

Existing resumable tests miss this because the harness inserts its "side writes" before the
collection scan and waits for them to majority-commit, so the scan also indexes them and the bulk
builder's multikey state masks the gap. The adjacent fix SERVER-127943 restored the bulk builder's
wildcard metadata flag on resume but did not touch the interceptor path.

Changes

• multi_index_block.cpp — _buildIndexStateInfo() now folds the interceptor's recorded
  multikey paths into the persisted isMultikey/multikeyPaths. On resume the merged state is
  restored into the bulk builder by the existing resume constructor, and the existing commit path
  marks the index multikey.
• index_build_block.h — adds a const getIndexBuildInterceptor() accessor (the block
  already owns the interceptor).
• multi_index_block_test.cpp — new regression test
  ResumeStateIncludesMultikeyFromSideWrites; findPersistedResumeState() gains an optional out
  parameter returning the parsed ResumeIndexInfo.
• jstests/noPassthrough/index_builds/resumable_index_build_multikey_from_side_writes.js —
  new end-to-end regression test covering shutdown/resume through a real mongod restart.

Why it's correct

• BulkBuilderImpl's resume constructor already restores isMultikey/multikeyPaths from
  IndexStateInfo, and MultiIndexBlock::commit() already calls setMultikey() whenever the
  bulk builder reports multikey — including for drain-writes-phase resumes. No new restore code.
• The change can only widen multikey state. A conservatively-multikey index is always correct
  (the planner is merely less aggressive); the pre-fix behavior is incorrect.
• Wildcard interaction is safe: side-write-driven isMultikey can enable the resume-derived
  _hasMultiKeyMetadataKeys, which only gates deduplication of adjacent metadata keys — a no-op
  when there are none.
• No IDL, wire-protocol, or on-disk format change; the persisted fields already exist
  (SERVER-50899), so downgraded binaries still parse the state document.

Testing

• bazel test //src/mongo/db/index_builds:multi_index_block_test
  • New: ResumeStateIncludesMultikeyFromSideWrites — asserts the persisted resume state carries
    isMultikey: true and path component 0 for a side-written array document; fails without the
    fix.
• python buildscripts/resmoke.py run --suites=no_passthrough jstests/noPassthrough/index_builds/resumable_index_build_multikey_from_side_writes.js
  • Asserts isMultiKey: true in explain, correct results for {a: {$gte: 4, $lte: 3}}, and a
    clean validate({full: true}) after resume; fails all three without the fix.
• Existing resumable_index_build_* suites continue to cover the bulk-builder multikey
  round-trip, including wildcard variants.

Risk

Low. One function on the shutdown-persist path plus a const accessor; failure direction is
strictly conservative; no format changes.