feat(ops): add ignore insufficient balance errors toggle and extract error constants

- Add 5th error filter switch IgnoreInsufficientBalanceErrors to suppress
  upstream insufficient balance / insufficient_quota errors from ops log
- Extract hardcoded error strings into package-level constants for
  shouldSkipOpsErrorLog, normalizeOpsErrorType, classifyOpsPhase, and
  classifyOpsIsBusinessLimited
- Define ErrNoAvailableAccounts sentinel error and replace all
  errors.New("no available accounts") call sites
- Update tests to use require.ErrorIs with the sentinel error

backend/internal/service/ops_settings.go

@@ -368,13 +368,14 @@ func defaultOpsAdvancedSettings() *OpsAdvancedSettings {
 		Aggregation: OpsAggregationSettings{
 			AggregationEnabled: false,
 		},
-		IgnoreCountTokensErrors:   true,  // count_tokens 404 是预期行为，默认忽略
-		IgnoreContextCanceled:     true,  // Default to true - client disconnects are not errors
-		IgnoreNoAvailableAccounts: false, // Default to false - this is a real routing issue
-		DisplayOpenAITokenStats:   false,
-		DisplayAlertEvents:        true,
-		AutoRefreshEnabled:        false,
-		AutoRefreshIntervalSec:    30,
+		IgnoreCountTokensErrors:         true,  // count_tokens 404 是预期行为，默认忽略
+		IgnoreContextCanceled:           true,  // Default to true - client disconnects are not errors
+		IgnoreNoAvailableAccounts:       false, // Default to false - this is a real routing issue
+		IgnoreInsufficientBalanceErrors: false, // 默认不忽略，余额不足可能需要关注
+		DisplayOpenAITokenStats:         false,
+		DisplayAlertEvents:              true,
+		AutoRefreshEnabled:              false,
+		AutoRefreshIntervalSec:          30,
 	}
 }