Extract safecsv writer for spreadsheet-safe exports

Wrap encoding/csv with formula-safe cell sanitization and
use it from log export streaming.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/iam/log_export_csv_test.go

@@ -33,16 +33,6 @@ func TestUniqueNonEmptyStrings(t *testing.T) {
 	assert.Equal(t, []string{"a", "b"}, got)
 }
 
-func TestCsvSafeCell(t *testing.T) {
-	t.Parallel()
-
-	assert.Equal(t, "plain", csvSafeCell("plain"))
-	assert.Equal(t, "'=1+1", csvSafeCell("=1+1"))
-	assert.Equal(t, "'+cmd", csvSafeCell("+cmd"))
-	assert.Equal(t, "'-2", csvSafeCell("-2"))
-	assert.Equal(t, "'@sum", csvSafeCell("@sum"))
-}
-
 func TestScimEmailFromUserName(t *testing.T) {
 	t.Parallel()