Use column schema in FDAlgorithm::GetKeys

Uses a more robust way of getting the number of columns. Avoids the
reliance on `input_table_` being set to the table being processed now,
indirectly inherited from the old implementation. Previously, there was
a possibility that `input_table_` would not be set to the table being
processed in `PliBasedFdAlgorithm` because of its `LoadData` overload,
but the only scenario which used it was using `TypoMiner`, and that
class would always set the option correctly.

src/algorithms/fd_algorithm.cpp

@@ -63,7 +63,14 @@ std::vector<Column const*> FDAlgorithm::GetKeys() const {
         }
     }
 
-    size_t const number_of_columns = input_table_->GetNumberOfColumns();
+    if (fds_count_per_col.empty()) return keys;
+    assert(fds_count_per_col.begin()->first->GetSchema() != nullptr);
+    size_t const number_of_columns = fds_count_per_col.begin()->first->GetSchema()->GetNumColumns();
+    assert(std::all_of(
+            ++fds_count_per_col.begin(), fds_count_per_col.end(),
+            [first_schema = fds_count_per_col.begin()->first->GetSchema()](auto const& kv_pair) {
+                return kv_pair.first->GetSchema() == first_schema;
+            }));
     for (auto const& [col, num]: fds_count_per_col) {
         if (num + 1 + cols_of_equal_values == number_of_columns) {
             keys.push_back(col);