GetInterchain: perChain + chains options (default first anchor per chain)

Transaction.GetInterchain and Block.GetInterchain take variadic options
(client.WithPerChain / client.WithChains, re-exported as sdk.WithPerChain /
sdk.WithChains) that map to the new prime-node ?perChain=&chains= query params.
Backward compatible: existing no-option calls get the default (one anchor per
chain). client.InterchainQuery builds the suffix; unit-tested.

transaction/interchain_test.go

@@ -0,0 +1,38 @@
+package transaction
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"git.dragonchain.com/dragonchain/prime-sdk-go/client"
+)
+
+// TestGetInterchainQuery confirms GetInterchain builds the request URI (path +
+// the perChain/chains query) end-to-end through the client.
+func TestGetInterchainQuery(t *testing.T) {
+	var gotURI string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotURI = r.URL.RequestURI()
+		_, _ = io.WriteString(w, `{"blockId":"42","validatorBlocks":[],"interchainTransactions":[]}`)
+	}))
+	defer srv.Close()
+
+	tc := NewTransactionClient(client.NewClient("pid", "kid", "key", srv.URL))
+
+	if _, err := tc.GetInterchain(context.Background(), "tx1", client.WithPerChain(2), client.WithChains("1", "0")); err != nil {
+		t.Fatalf("GetInterchain with opts: %v", err)
+	}
+	if want := "/api/v1/transaction/tx1/interchain?perChain=2&chains=1,0"; gotURI != want {
+		t.Errorf("with opts: URI = %q, want %q", gotURI, want)
+	}
+
+	if _, err := tc.GetInterchain(context.Background(), "tx1"); err != nil {
+		t.Fatalf("GetInterchain default: %v", err)
+	}
+	if want := "/api/v1/transaction/tx1/interchain"; gotURI != want {
+		t.Errorf("default (no opts): URI = %q, want %q", gotURI, want)
+	}
+}