fix: improve CSV parser error handling

lib/mv/membership/import/csv_parser.ex

@@ -23,13 +23,9 @@ defmodule Mv.Membership.Import.CsvParser do
 
   @spec parse(binary()) :: {:ok, row(), [numbered_row()]} | {:error, String.t()}
   def parse(file_content) when is_binary(file_content) do
-    content =
-      file_content
-      |> strip_bom()
-      |> normalize_line_endings()
-
-    with :ok <- validate_content_not_empty(content),
-         :ok <- check_balanced_quotes(content),
+    with :ok <- validate_utf8(file_content),
+         content <- file_content |> strip_bom() |> normalize_line_endings(),
+         :ok <- validate_content_not_empty(content),
          {:ok, header_record, data_records} <- extract_header_and_data(content),
          :ok <- validate_header_not_empty(header_record),
          {:ok, headers, rows} <- parse_csv_records(header_record, data_records) do
@@ -41,6 +37,15 @@ defmodule Mv.Membership.Import.CsvParser do
 
   def parse(_), do: {:error, "Invalid CSV content"}
 
+  @spec validate_utf8(binary()) :: :ok | {:error, String.t()}
+  defp validate_utf8(content) do
+    if String.valid?(content) do
+      :ok
+    else
+      {:error, "CSV must be valid UTF-8"}
+    end
+  end
+
   @spec validate_content_not_empty(binary()) :: :ok | {:error, String.t()}
   defp validate_content_not_empty(content) do
     if String.trim(content) == "" do
@@ -157,10 +162,10 @@ defmodule Mv.Membership.Import.CsvParser do
     try do
       rows =
         data_records
-        |> Enum.reduce([], fn {line_no, record}, acc ->
+        |> Enum.reduce_while([], fn {line_no, record}, acc ->
           case String.trim(record) do
             "" ->
-              acc
+              {:cont, acc}
 
             _ ->
               parsed = parser.parse_string(ensure_trailing_newline(record), skip_headers: false)
@@ -168,20 +173,23 @@ defmodule Mv.Membership.Import.CsvParser do
               case parsed do
                 [row] when is_list(row) ->
                   if empty_row?(row) do
-                    acc
+                    {:cont, acc}
                   else
-                    [{line_no, row} | acc]
+                    {:cont, [{line_no, row} | acc]}
                   end
 
-                # empty row or unparsable row -> treat as skipped empty row
+                # unparsable row -> return error with line number
                 _ ->
-                  acc
+                  snippet = String.slice(record, 0, min(50, String.length(record)))
+                  {:halt, {:error, "Failed to parse CSV data at line #{line_no}: #{inspect(snippet)}"}}
               end
           end
         end)
-        |> Enum.reverse()
 
-      {:ok, rows}
+      case rows do
+        {:error, reason} -> {:error, reason}
+        rows -> {:ok, Enum.reverse(rows)}
+      end
     rescue
       e ->
         {:error, "Failed to parse CSV: #{Exception.message(e)}"}
@@ -193,21 +201,6 @@ defmodule Mv.Membership.Import.CsvParser do
     Enum.all?(row, fn field -> String.trim(field) == "" end)
   end
 
-  # Check if quotes are balanced in the content.
-  #
-  # This is a simple check that counts quote characters. In CSV, escaped quotes
-  # are represented as "", so an odd number of quotes indicates unbalanced quotes.
-  @spec check_balanced_quotes(binary()) :: :ok | {:error, String.t()}
-  defp check_balanced_quotes(content) do
-    quote_count = content |> :binary.bin_to_list() |> Enum.count(&(&1 == @quote))
-
-    if rem(quote_count, 2) != 0 do
-      {:error, "Unbalanced quotes in CSV file"}
-    else
-      :ok
-    end
-  end
-
   # --- Record splitting with correct line numbers (quote-aware) ---
   #
   # This splits the CSV into "records" separated by newline *outside quotes*,