PIL-2854 Add amendMultipleAccountingPeriods feature flag (#644)

## Summary
Adds feature flag `amendMultipleAccountingPeriods` for the Change
Accounting period feature (V2 endpoints for Display Subscription and
Amend Subscription).

## Changes
- **conf/application.conf**: Add `amendMultipleAccountingPeriods =
false` under `features`
- **app/config/FrontendAppConfig.scala**: Expose
`amendMultipleAccountingPeriods` for injection in controllers/connector
- **test/config/FrontendAppConfigSpec.scala**: Test that flag defaults
to false

## Acceptance criteria
- [x] Flag exists in config, defaults to false
- [x] Flag can be toggled per environment (same mechanism as other
feature flags)
- [x] Flag is injectable into controllers that handle accounting period
display and amendment flows (via existing FrontendAppConfig injection)

## Out of scope (follow-up)
- V2 endpoint switching logic; this ticket is feature-flag setup only.

Co-authored-by: Mannkaur2 <211959528+Mannkaur2@users.noreply.github.com>

Author: olakunlekuye2

app/config/FrontendAppConfig.scala

@@ -102,10 +102,11 @@ class FrontendAppConfig @Inject() (configuration: Configuration, servicesConfig:
   val researchUrl:                  String  = configuration.get[String]("urls.researchUrl")
   val howToPayPillar2TaxesUrl:      String  = configuration.get[String]("urls.howToPayPillar2Taxes")
 
-  val opsBaseUrl:             String  = servicesConfig.baseUrl("ops")
-  val opsStartUrl:            String  = configuration.get[String]("microservice.services.ops.startUrl")
-  val enablePayByBankAccount: Boolean = configuration.get[Boolean]("features.enablePayByBankAccount")
-  val useAccountActivityApi:  Boolean = configuration.get[Boolean]("features.useAccountActivityApi")
+  val opsBaseUrl:                     String  = servicesConfig.baseUrl("ops")
+  val opsStartUrl:                    String  = configuration.get[String]("microservice.services.ops.startUrl")
+  val enablePayByBankAccount:         Boolean = configuration.get[Boolean]("features.enablePayByBankAccount")
+  val useAccountActivityApi:          Boolean = configuration.get[Boolean]("features.useAccountActivityApi")
+  val amendMultipleAccountingPeriods: Boolean = configuration.get[Boolean]("features.amendMultipleAccountingPeriods")
 
   def transactionHistoryEndDate: LocalDate = {
     val date = configuration.get[String]("features.transactionHistoryEndDate")

conf/application.conf

@@ -173,6 +173,7 @@ features {
   btaAccessEnabled = true
   enablePayByBankAccount = true
   useAccountActivityApi = false
+  amendMultipleAccountingPeriods = false
 
   incorporatedEntityBvEnabled = false
   partnershipBvEnabled = false

test/config/FrontendAppConfigSpec.scala

@@ -124,6 +124,12 @@ class FrontendAppConfigSpec extends SpecBase with TableDrivenPropertyChecks {
       }
     }
 
+    ".amendMultipleAccountingPeriods" must {
+      "return false by default" in {
+        config.amendMultipleAccountingPeriods mustBe false
+      }
+    }
+
     ".maxDaysAgoToConsiderPaymentAsRecent" must {
       "read value from config" in {
         config.maxDaysAgoToConsiderPaymentAsRecent mustBe 60