fix(backup): prevent duplicate predicates in backup manifest for vector indexes (#9679)

**Description**

Fixed a bug in ProcessBackupRequest where vecPredMap[gid] was being
built using predMap[gid] as the append base instead of vecPredMap[gid].
This caused all base predicates in the group to be duplicated in the
manifest every time a vector predicate was found, making manifest.json
grow larger and larger with each backup.
With multiple vector predicates in the same group, each iteration also
overwrote the previous result, so only the last vector predicate's
supporting entries (__vector_entry, __vector_, __vector_dead) were kept
the rest were silently lost.

---------

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

Author: shiva-istari

dgraphtest/local_cluster.go

@@ -1397,6 +1397,27 @@ func (c *LocalCluster) GetAlphaGrpcEndpoint(id int) (string, error) {
 
 // CopyExportToHost copies exported files from the container to a host directory.
 // It returns the paths to RDF/JSON files and schema files on the host.
+func (c *LocalCluster) ReadFileFromContainer(containerPath string) ([]byte, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), requestTimeout)
+	defer cancel()
+
+	ts, _, err := c.dcli.CopyFromContainer(ctx, c.alphas[0].cid(), containerPath)
+	if err != nil {
+		return nil, errors.Wrapf(err, "error copying file from container [%v]", c.alphas[0].cname())
+	}
+	defer ts.Close()
+
+	tr := tar.NewReader(ts)
+	if _, err := tr.Next(); err != nil {
+		return nil, errors.Wrap(err, "error reading tar header")
+	}
+	data, err := io.ReadAll(tr)
+	if err != nil {
+		return nil, errors.Wrap(err, "error reading file contents from tar stream")
+	}
+	return data, nil
+}
+
 func (c *LocalCluster) CopyExportToHost(exportDir, hostDir string) (dataFiles, schemaFiles []string, err error) {
 	ctx, cancel := context.WithTimeout(context.Background(), requestTimeout)
 	defer cancel()