refactor(client): wrap all external requests in retry transport

backend/internal/zarinhub/client.go

@@ -54,14 +54,48 @@ func NewClient(
 		logger = loggers[0]
 	}
 
-	return &Client{
+	if httpClient == nil {
+		httpClient = http.DefaultClient
+	}
+
+	client := &Client{
 		httpClient:    httpClient,
 		endpointURL:   endpointURL,
 		tokenProvider: tokenProvider,
 		maxRetries:    maxRetries,
-		retryWait:     waitForRetry,
 		logger:        logger,
 	}
+
+	if _, ok := httpClient.Transport.(*RetryTransport); !ok && maxRetries > 0 {
+		rt := NewRetryTransport(
+			httpClient.Transport,
+			httpClient.Timeout,
+			maxRetries,
+			logger,
+		)
+		rt.retryWait = func(ctx context.Context, delay time.Duration) bool {
+			if client.retryWait != nil {
+				return client.retryWait(ctx, delay)
+			}
+			return waitForRetry(ctx, delay)
+		}
+		client.httpClient = &http.Client{
+			Transport: rt,
+		}
+	} else if rt, ok := httpClient.Transport.(*RetryTransport); ok {
+		origWait := rt.retryWait
+		rt.retryWait = func(ctx context.Context, delay time.Duration) bool {
+			if client.retryWait != nil {
+				return client.retryWait(ctx, delay)
+			}
+			if origWait != nil {
+				return origWait(ctx, delay)
+			}
+			return waitForRetry(ctx, delay)
+		}
+	}
+
+	return client
 }
 
 func (c *Client) log() *slog.Logger {
@@ -110,88 +144,7 @@ func (c *Client) Convert(
 	ctx context.Context,
 	cardNumber string,
 ) (service.ConversionResult, error) {
-	var (
-		result service.ConversionResult
-		err    error
-	)
-
-	waitFn := c.retryWait
-	if waitFn == nil {
-		waitFn = waitForRetry
-	}
-
-	for attempt := 0; attempt <= c.maxRetries; attempt++ {
-		if attempt > 0 {
-			c.log().InfoContext(
-				ctx,
-				fmt.Sprintf("[RETRY] zarinhub request attempt %d", attempt),
-				"is_retry", true,
-				"retry_count", attempt,
-			)
-		} else {
-			c.log().InfoContext(
-				ctx,
-				"[INITIAL] zarinhub request",
-				"is_retry", false,
-				"retry_count", 0,
-			)
-		}
-
-		result, err = c.convertOnce(
-			ctx,
-			cardNumber,
-		)
-
-		if err == nil {
-			return result, nil
-		}
-
-		// Don't retry permanent/non-transient errors.
-		if !isRetryable(err) {
-			return result, err
-		}
-
-		// We already used all configured retries.
-		if attempt == c.maxRetries {
-			if attempt > 0 {
-				c.log().ErrorContext(
-					ctx,
-					"[RETRY] zarinhub request failed: all retries exhausted",
-					"is_retry", true,
-					"retry_count", attempt,
-					"error", err,
-				)
-			}
-			return result, err
-		}
-
-		// If the caller canceled the request, stop immediately.
-		if ctx.Err() != nil {
-			if errors.Is(ctx.Err(), context.Canceled) {
-				return result, fmt.Errorf("zarinhub request canceled: %w", ctx.Err())
-			}
-			return result, err
-		}
-
-		delay := retryDelay(attempt)
-		c.log().WarnContext(
-			ctx,
-			fmt.Sprintf("[RETRY] zarinhub request failed, retrying in %v (retry %d)", delay, attempt+1),
-			"is_retry", true,
-			"next_retry", attempt + 1,
-			"delay_ms", delay.Milliseconds(),
-			"error", err,
-		)
-
-		if !waitFn(ctx, delay) {
-			if errors.Is(ctx.Err(), context.Canceled) {
-				return result, fmt.Errorf("zarinhub request canceled: %w", ctx.Err())
-			}
-			return result, err
-		}
-	}
-
-	return result, err
+	return c.convertOnce(ctx, cardNumber)
 }
 
 func (c *Client) convertOnce(