core/txpool/blobpool: fall back to pool in GetCells for blob-mode cache entries (#35439)

## Description

Blob mode cache entries (the default until a CL advertises
`engine_getBlobsV4` via `engine_exchangeCapabilities`) carry no cells,
but `Cache.GetCells` treated any entry as a hit and served null cells
from the empty custody set instead of falling back to the pool.

Found via the EELS execute-blobs simulator (EIP-8070 `engine_getBlobsV4`
tests), which never calls `engine_exchangeCapabilities`: null cells
appeared intermittently depending on the race with the 1s top-K preload.
With the fix the suite passes 139/139 (from 87/139 on master).

core/txpool/blobpool/cache.go

@@ -265,7 +265,10 @@ func (c *Cache) GetCells(vhashes []common.Hash, mask types.CustodyBitmap) ([][]*
 	c.mu.Lock()
 	for vhash, idxs := range indices {
 		cached, ok := c.entries[vhash]
-		if !ok {
+		if !ok || cached.cell == nil {
+			// Entries loaded in blob mode carry no cells; fall back to
+			// the blobpool instead of serving null cells for a blob
+			// that is actually available.
 			misses = append(misses, vhash)
 			continue
 		}

core/txpool/blobpool/cache_test.go

@@ -283,6 +283,36 @@ func TestCacheGetBlobs(t *testing.T) {
 	tc.expectEntries(t, tc.vhashes[1]...)
 }
 
+// TestCacheGetCellsBlobModeFallback checks that GetCells falls back to the
+// blobpool for entries that were cached in blob mode (without cells) instead
+// of serving null cells for a blob that is available in the pool.
+func TestCacheGetCellsBlobModeFallback(t *testing.T) {
+	tc := newTestCache(t, []txSpec{
+		{blobs: 1, tip: 300},
+	})
+	tc.expectEntries(t, tc.vhashes[0]...) // blob-mode entries, no cells
+
+	cells, proofs, err := tc.GetCells(tc.vhashes[0], types.CustodyBitmapAll)
+	if err != nil {
+		t.Fatalf("GetCells: %v", err)
+	}
+	for i := range cells {
+		if len(cells[i]) == 0 {
+			t.Errorf("no cells returned for blob %d", i)
+		}
+		for j, cell := range cells[i] {
+			if cell == nil {
+				t.Errorf("cell %d of blob %d missing in GetCells response", j, i)
+			}
+		}
+		for j, proof := range proofs[i] {
+			if proof == nil {
+				t.Errorf("proof %d of blob %d missing in GetCells response", j, i)
+			}
+		}
+	}
+}
+
 // TestCacheTopKRefresh verifies that when a more profitable tx appears in the
 // pool, the next topK tick replaces the cached entry with the better one.
 func TestCacheTopKRefresh(t *testing.T) {