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/provider/plaid_test.rb

@@ -0,0 +1,80 @@
+require "test_helper"
+
+class Provider::PlaidTest < ActiveSupport::TestCase
+  setup do
+    # Do not change, this is whitelisted in the Plaid Dashboard for local dev
+    @redirect_url = "http://localhost:3000/accounts"
+
+    # A specialization of Plaid client with sandbox-only extensions
+    @plaid = Provider::PlaidSandbox.new
+  end
+
+  test "gets link token" do
+    VCR.use_cassette("plaid/link_token") do
+      link_token = @plaid.get_link_token(
+        user_id: "test-user-id",
+        webhooks_url: "https://example.com/webhooks",
+        redirect_url: @redirect_url
+      )
+
+      assert_match /link-sandbox-.*/, link_token.link_token
+    end
+  end
+
+  test "exchanges public token" do
+    VCR.use_cassette("plaid/exchange_public_token") do
+      public_token = @plaid.create_public_token
+      exchange_response = @plaid.exchange_public_token(public_token)
+
+      assert_match /access-sandbox-.*/, exchange_response.access_token
+    end
+  end
+
+  test "gets item" do
+    VCR.use_cassette("plaid/get_item") do
+      access_token = get_access_token
+      item = @plaid.get_item(access_token).item
+
+      assert_equal "ins_109508", item.institution_id
+      assert_equal "First Platypus Bank", item.institution_name
+    end
+  end
+
+  test "gets item accounts" do
+    VCR.use_cassette("plaid/get_item_accounts") do
+      access_token = get_access_token
+      accounts_response = @plaid.get_item_accounts(access_token)
+
+      assert_equal 4, accounts_response.accounts.size
+    end
+  end
+
+  test "gets item investments" do
+    VCR.use_cassette("plaid/get_item_investments") do
+      access_token = get_access_token
+      investments_response = @plaid.get_item_investments(access_token)
+
+      assert_equal 3, investments_response.holdings.size
+      assert_equal 4, investments_response.transactions.size
+    end
+  end
+
+  test "gets item liabilities" do
+    VCR.use_cassette("plaid/get_item_liabilities") do
+      access_token = get_access_token
+      liabilities_response = @plaid.get_item_liabilities(access_token)
+
+      assert liabilities_response.credit.count > 0
+      assert liabilities_response.student.count > 0
+    end
+  end
+
+  private
+    def get_access_token
+      VCR.use_cassette("plaid/access_token") do
+        public_token = @plaid.create_public_token
+        exchange_response = @plaid.exchange_public_token(public_token)
+        exchange_response.access_token
+      end
+    end
+end