feat(bulk): add HNSW vector index support to bulk loader (#9598)

**Description**
Adds HNSW vector index support to Dgraph's bulk loader, enabling bulk
loading of vector predicates with automatic index construction.

**Checklist**

- [x] The PR title follows the
[Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/#summary) syntax,
leading
      with `fix:`, `feat:`, `chore:`, `ci:`, etc.
- [x] Code compiles correctly and linting (via trunk) passes locally
- [x] Tests added for new functionality, or regression tests for bug
fixes added as applicable
- [ ] For public APIs, new features, etc., a PR on the
[docs repo](https://github.com/dgraph-io/dgraph-docs) staged and linked
here. This process can
be simplified by going to the [public docs
site](https://docs.dgraph.io/) and clicking the
"Edit this page" button at the bottom of page(s) relevant to your
changes. Ensure that you
indicate in the PR that this is an **unreleased** feature so that it
does not get merged into
      the main docs prematurely.

---------

Co-authored-by: Shiva <shiva@Shivajis-MacBook-Pro.local>

Author: shiva-istari

dgraph/cmd/bulk/loader.go

@@ -97,6 +97,7 @@ type state struct {
 	mapFileId     uint32 // Used atomically to name the output files of the mappers.
 	dbs           []*badger.DB
 	tmpDbs        []*badger.DB // Temporary DB to write the split lists to avoid ordering issues.
+	vectorTmpDb   *badger.DB   // Shared temporary DB for vector data and HNSW index building.
 	writeTs       uint64       // All badger writes use this timestamp
 	namespaces    *sync.Map    // To store the encountered namespaces.
 	errorLog      *errorLogger // Error logger for --log_errors
@@ -586,6 +587,11 @@ func (ld *loader) cleanup() {
 		x.Check(db.Close())
 		x.Check(os.RemoveAll(opts.Dir))
 	}
+	if ld.vectorTmpDb != nil {
+		opts := ld.vectorTmpDb.Opts()
+		x.Check(ld.vectorTmpDb.Close())
+		x.Check(os.RemoveAll(opts.Dir))
+	}
 	if ld.errorLog != nil {
 		if err := ld.errorLog.Close(); err != nil {
 			glog.Warningf("error closing error log: %v", err)