feat: 체인 클라이언트 멀티노드 페일오버 (SPOF 해결)

CHAIN_NODE_URLS 환경변수(쉼표 구분)로 복수 노드 지정 가능.
Client.Call()이 네트워크/HTTP 오류 시 다음 노드로 자동 전환.
RPC 레벨 오류(트랜잭션 실패 등)는 즉시 반환 (페일오버 미적용).
기존 CHAIN_NODE_URL 단일 설정은 하위 호환 유지.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/chain/client.go

@@ -34,21 +34,55 @@ func (e *rpcError) Error() string {
 }
 
 // Client is a JSON-RPC 2.0 client for the TOL Chain node.
+// It supports multiple node URLs for failover: on a network/HTTP error the
+// client automatically retries against the next URL in the list.
+// RPC-level errors (transaction failures, etc.) are returned immediately
+// without failover since they indicate a logical error, not node unavailability.
 type Client struct {
-	nodeURL string
-	http    *http.Client
-	idSeq   atomic.Int64
+	nodeURLs []string
+	http     *http.Client
+	idSeq    atomic.Int64
+	next     atomic.Uint64 // round-robin index
 }
 
-func NewClient(nodeURL string) *Client {
+// NewClient creates a client for one or more chain node URLs.
+// When multiple URLs are provided, failed requests fall over to the next URL.
+func NewClient(nodeURLs ...string) *Client {
+	if len(nodeURLs) == 0 {
+		panic("chain.NewClient: at least one node URL is required")
+	}
 	return &Client{
-		nodeURL: nodeURL,
-		http:    &http.Client{Timeout: 10 * time.Second},
+		nodeURLs: nodeURLs,
+		http:     &http.Client{Timeout: 10 * time.Second},
 	}
 }
 
 // Call invokes a JSON-RPC method and unmarshals the result into out.
+// On network or HTTP errors it tries each node URL once before giving up.
 func (c *Client) Call(method string, params any, out any) error {
+	n := len(c.nodeURLs)
+	start := int(c.next.Load() % uint64(n))
+
+	var lastErr error
+	for i := 0; i < n; i++ {
+		url := c.nodeURLs[(start+i)%n]
+		err := c.callNode(url, method, params, out)
+		if err == nil {
+			return nil
+		}
+		// RPC-level error (e.g. tx execution failure): return immediately,
+		// retrying on another node would give the same result.
+		if _, isRPC := err.(*rpcError); isRPC {
+			return err
+		}
+		// Network / HTTP error: mark this node as degraded and try the next.
+		lastErr = err
+		c.next.Add(1)
+	}
+	return fmt.Errorf("all chain nodes unreachable: %w", lastErr)
+}
+
+func (c *Client) callNode(nodeURL, method string, params any, out any) error {
 	reqBody := rpcRequest{
 		JSONRPC: "2.0",
 		ID:      c.idSeq.Add(1),
@@ -60,14 +94,14 @@ func (c *Client) Call(method string, params any, out any) error {
 		return fmt.Errorf("marshal RPC request: %w", err)
 	}
 
-	resp, err := c.http.Post(c.nodeURL, "application/json", bytes.NewReader(data))
+	resp, err := c.http.Post(nodeURL, "application/json", bytes.NewReader(data))
 	if err != nil {
-		return fmt.Errorf("RPC network error: %w", err)
+		return fmt.Errorf("RPC network error (%s): %w", nodeURL, err)
 	}
 	defer resp.Body.Close()
 
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-		return fmt.Errorf("RPC HTTP error: status %d", resp.StatusCode)
+		return fmt.Errorf("RPC HTTP error (%s): status %d", nodeURL, resp.StatusCode)
 	}
 
 	body, err := io.ReadAll(io.LimitReader(resp.Body, 10*1024*1024))