fix(ops): use normalized error type for all classification functions

- Compute normalizedType once and pass to classifyOpsPhase,
  classifyOpsSeverity, classifyOpsIsBusinessLimited, classifyOpsIsRetryable
  instead of raw parsed.ErrorType
- Add test case verifying known type takes precedence over conflicting code

Addresses Copilot review feedback on PR #680.

backend/internal/handler/ops_error_logger_test.go

@@ -263,6 +263,9 @@ func TestNormalizeOpsErrorType(t *testing.T) {
 		{"empty type with balance code", "", "INSUFFICIENT_BALANCE", "billing_error"},
 		{"empty type with subscription code", "", "SUBSCRIPTION_NOT_FOUND", "subscription_error"},
 		{"empty type no code", "", "", "api_error"},
+
+		// Known type overrides conflicting code-based mapping.
+		{"known type overrides conflicting code", "rate_limit_error", "INSUFFICIENT_BALANCE", "rate_limit_error"},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {