Plaid sync domain improvements (#2267)

Breaks our Plaid sync process out into more manageable classes. Notably, this moves the sync process to a distinct, 2-step flow:

1. Import stage - we first make API calls and import Plaid data to "mirror" tables
2. Processing stage - read the raw data, apply business rules, build internal domain models and sync balances

This provides several benefits:

- Plaid syncs can now be "replayed" without fetching API data again
- Mirror tables provide better audit and debugging capabilities
- Eliminates the "all or nothing" sync behavior that is currently in place, which is brittle

test/models/plaid_account/importer_test.rb

@@ -0,0 +1,35 @@
+require "test_helper"
+
+class PlaidAccount::ImporterTest < ActiveSupport::TestCase
+  setup do
+    @mock_provider = PlaidMock.new
+    @plaid_account = plaid_accounts(:one)
+    @plaid_item = @plaid_account.plaid_item
+
+    @accounts_snapshot = PlaidItem::AccountsSnapshot.new(@plaid_item, plaid_provider: @mock_provider)
+    @account_snapshot = @accounts_snapshot.get_account_data(@plaid_account.plaid_id)
+  end
+
+  test "imports account data" do
+    PlaidAccount::Importer.new(@plaid_account, account_snapshot: @account_snapshot).import
+
+    assert_equal @account_snapshot.account_data.account_id, @plaid_account.plaid_id
+    assert_equal @account_snapshot.account_data.name, @plaid_account.name
+    assert_equal @account_snapshot.account_data.mask, @plaid_account.mask
+    assert_equal @account_snapshot.account_data.type, @plaid_account.plaid_type
+    assert_equal @account_snapshot.account_data.subtype, @plaid_account.plaid_subtype
+
+    # This account has transactions data
+    assert_equal PlaidMock::TRANSACTIONS.count, @plaid_account.raw_transactions_payload["added"].count
+
+    # This account does not have investment data
+    assert_equal 0, @plaid_account.raw_investments_payload["holdings"].count
+    assert_equal 0, @plaid_account.raw_investments_payload["securities"].count
+    assert_equal 0, @plaid_account.raw_investments_payload["transactions"].count
+
+    # This account is a credit card, so it should have liability data
+    assert_equal @plaid_account.plaid_id, @plaid_account.raw_liabilities_payload["credit"]["account_id"]
+    assert_nil @plaid_account.raw_liabilities_payload["mortgage"]
+    assert_nil @plaid_account.raw_liabilities_payload["student"]
+  end
+end