fix(test): skip ethereum mainnet integration when RPC URL is missing

Local runs of go test -tags=integration ./... now skip rather than fail
when ETHEREUM_MAINNET_RPC_URL is not configured. CI still sets the var, so
behavior there is unchanged. Also clarify that blockByNumber returns a
header-only block.

Author: piotr-roslaniec

pkg/chain/ethereum/ethereum.go

@@ -450,6 +450,8 @@ func (bc *baseChain) blockByNumber(number uint64) (*types.Block, error) {
 
 	// Fetch the header to avoid decoding full transactions (some providers
 	// may return transaction types the client library does not support yet).
+	// The returned *types.Block carries only header fields; transactions and
+	// uncles are empty. Callers that need tx data must fetch the full block.
 	header, err := bc.client.HeaderByNumber(ctx, big.NewInt(int64(number)))
 	if err != nil {
 		return nil, err

pkg/chain/ethereum/ethereum_integration_test.go

@@ -21,7 +21,7 @@ import (
 func TestBaseChain_GetBlockNumberByTimestamp(t *testing.T) {
 	ethereumURL := os.Getenv("ETHEREUM_MAINNET_RPC_URL")
 	if ethereumURL == "" {
-		t.Fatal("ETHEREUM_MAINNET_RPC_URL not set")
+		t.Skip("ETHEREUM_MAINNET_RPC_URL not set; skipping mainnet integration test")
 	}
 
 	client, err := ethclient.Dial(ethereumURL)