Fix statedb errors silently swallowed in multi-key queries

[Atishyy27]

What's wrong

GetStateMultipleKeys and GetPrivateDataMultipleKeys in
core/ledger/kvledger/txmgmt/txmgr/query_executor.go both discard the
underlying statedb error and return (nil, nil) instead of propagating it:

versionedValues, err := q.txmgr.db.GetStateMultipleKeys(ns, keys)
if err != nil {
    return nil, nil   // should be: return nil, err
}

The identical pattern appears in GetPrivateDataMultipleKeys a few lines
down. Every other error path in this file correctly does return nil, err
(see GetPrivateDataMetadataByHash right above it, for example) — these two
are the only ones that swallow it.

Why it matters

If the state database fails while resolving a multi-key read (as invoked
from chaincode via these two query executor methods), the ledger currently
reports success with an empty/nil result instead of surfacing the failure.
A caller has no way to distinguish "no data" from "the read actually
failed," which can let a chaincode invocation proceed on the wrong
assumption.

Fix

Propagate the error on both paths, matching every other branch in the file.

Testing

Added a regression test for each function
(TestGetStateMultipleKeysPropagatesDBError,
TestGetPrivateDataMultipleKeysPropagatesDBError) using a minimal
statedb.VersionedDB fake that returns a fixed error, wired through
privacyenabledstate.DB so the real query_executor code path is
exercised end to end (not the underlying leveldb/couchdb implementation).

Verified both tests fail against the pre-fix code (asserting an error is
returned, they got nil instead) and pass after the two-line fix.

go test ./core/ledger/kvledger/txmgmt/txmgr/... -run "TestGetStateMultipleKeysPropagatesDBError|TestGetPrivateDataMultipleKeysPropagatesDBError" -v
--- PASS: TestGetStateMultipleKeysPropagatesDBError (0.00s)
--- PASS: TestGetPrivateDataMultipleKeysPropagatesDBError (0.00s)

go build ./... and gofmt -l both clean on the changed files.