fix(github-graphql): prevent panic in graphql rate limit polling goroutine

Replace panic in GraphqlAsyncClient rate-limit polling goroutine with graceful error handling.

Previously, any error while fetching rate limit (e.g., transient network issues or 401 responses) would trigger a panic inside a background goroutine, crashing the entire DevLake process.

Now, errors are logged and the client retries in the next cycle while retaining the last known rate limit.

Design decisions:
- Avoid panic in background goroutines: rate-limit polling is non-critical and should not bring down the entire pipeline.
- Use last known rateRemaining on runtime failures instead of resetting or blocking, ensuring continued progress with eventual consistency.
- Retry via existing polling mechanism instead of immediate retry to prevent tight retry loops and unnecessary API pressure.
- Introduce a default fallback (5000) only for initial rate-limit fetch failures, since no prior state exists at startup.
- Separate handling of initial vs runtime failures:
  - Initial failure → fallback to default (5000)
  - Runtime failure → retain previous value

Fixes #8788 (bug 1)

Author: Ke-vin-S

backend/helpers/pluginhelper/api/graphql_async_client.go

@@ -47,6 +47,8 @@ type GraphqlAsyncClient struct {
 	getRateCost      func(q interface{}) int
 }
 
+const defaultRateLimit = 5000
+
 // CreateAsyncGraphqlClient creates a new GraphqlAsyncClient
 func CreateAsyncGraphqlClient(
 	taskCtx plugin.TaskContext,
@@ -68,9 +70,11 @@ func CreateAsyncGraphqlClient(
 	if getRateRemaining != nil {
 		rateRemaining, resetAt, err := getRateRemaining(taskCtx.GetContext(), graphqlClient, logger)
 		if err != nil {
-			panic(err)
+			graphqlAsyncClient.logger.Warn(err, "failed to fetch initial graphql rate limit, fallback to default")
+			graphqlAsyncClient.updateRateRemaining(defaultRateLimit, nil)
+		} else {
+			graphqlAsyncClient.updateRateRemaining(rateRemaining, resetAt)
 		}
-		graphqlAsyncClient.updateRateRemaining(rateRemaining, resetAt)
 	}
 
 	// load retry/timeout from configuration
@@ -126,7 +130,9 @@ func (apiClient *GraphqlAsyncClient) updateRateRemaining(rateRemaining int, rese
 		case <-time.After(nextDuring):
 			newRateRemaining, newResetAt, err := apiClient.getRateRemaining(apiClient.ctx, apiClient.client, apiClient.logger)
 			if err != nil {
-				panic(err)
+				apiClient.logger.Warn(err, "failed to update graphql rate limit, will retry next cycle")
+				apiClient.updateRateRemaining(apiClient.rateRemaining, nil)
+				return
 			}
 			apiClient.updateRateRemaining(newRateRemaining, newResetAt)
 		}