eth/protocols/snap: serve snap requests when possible (#25644)

This PR makes it so that the snap server responds to trie heal requests when possible, even if the snapshot does not exist. The idea being that it might prolong the lifetime of a state root, so we don't have to pivot quite as often.

cmd/devp2p/internal/ethtest/snap.go

@@ -406,8 +406,10 @@ func (s *Suite) TestSnapTrieNodes(t *utesting.T) {
 				{[]byte{0}},
 				{[]byte{1}, []byte{0}},
 			},
-			nBytes:    5000,
-			expHashes: []common.Hash{},
+			nBytes: 5000,
+			expHashes: []common.Hash{
+				common.HexToHash("0x1ee1bb2fbac4d46eab331f3e8551e18a0805d084ed54647883aa552809ca968d"),
+			},
 		},
 		{
 			// The leaf is only a couple of levels down, so the continued trie traversal causes lookup failures.
@@ -437,7 +439,35 @@ func (s *Suite) TestSnapTrieNodes(t *utesting.T) {
 				common.HexToHash("0xbcefee69b37cca1f5bf3a48aebe08b35f2ea1864fa958bb0723d909a0e0d28d8"),
 			},
 		},
-	} {
+		{
+			/*
+				A test against this account, requesting trie nodes for the storage trie
+				{
+				  "balance": "0",
+				  "nonce": 1,
+				  "root": "0xbe3d75a1729be157e79c3b77f00206db4d54e3ea14375a015451c88ec067c790",
+				  "codeHash": "0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470",
+				  "storage": {
+				    "0x405787fa12a823e0f2b7631cc41b3ba8828b3321ca811111fa75cd3aa3bb5ace": "02",
+				    "0xb10e2d527612073b26eecdfd717e6a320cf44b4afac2b0732d9fcbe2b7fa0cf6": "01",
+				    "0xc2575a0e9e593c00f959f8c92f12db2869c3395a3b0502d05e2516446f71f85b": "03"
+				  },
+				  "key": "0xf493f79c43bd747129a226ad42529885a4b108aba6046b2d12071695a6627844"
+				}
+			*/
+			root: s.chain.RootAt(999),
+			paths: []snap.TrieNodePathSet{
+				{
+					common.FromHex("0xf493f79c43bd747129a226ad42529885a4b108aba6046b2d12071695a6627844"),
+					[]byte{0},
+				},
+			},
+			nBytes: 5000,
+			expHashes: []common.Hash{
+				common.HexToHash("0xbe3d75a1729be157e79c3b77f00206db4d54e3ea14375a015451c88ec067c790"),
+			},
+		},
+	}[7:] {
 		tc := tc
 		if err := s.snapGetTrieNodes(t, &tc); err != nil {
 			t.Errorf("test %d \n #hashes %x\n root: %#x\n bytes: %d\nfailed: %v", i, len(tc.expHashes), tc.root, tc.nBytes, err)