Refactor getBillingPeriodInDays to support all values (#6169)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1201807753394693/task/1210432795362321?focus=true

### Description
Refactor getBillingPeriodInDays() so we support all Google Play date
ranges

### Steps to test this PR

_Free Trial purchase_
- [x] Set a US account and make sure you are Privacy Pro eligible
- [x] Go to paywall
- [x] Check everything is fine and free trial is offered
- [x] Purchase free trial

_Unit tests_
- [x] Check Ci is green ✅ 

### No UI changes

Author: nalcalag

subscriptions/subscriptions-impl/src/main/java/com/duckduckgo/subscriptions/impl/SubscriptionsManager.kt

@@ -81,6 +81,8 @@ import dagger.SingleInstanceIn
 import java.io.IOException
 import java.time.Duration
 import java.time.Instant
+import java.time.Period
+import java.time.format.DateTimeParseException
 import javax.inject.Inject
 import kotlin.time.Duration.Companion.milliseconds
 import kotlinx.coroutines.CoroutineScope
@@ -95,6 +97,7 @@ import kotlinx.coroutines.flow.onSubscription
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 import logcat.LogPriority.ERROR
+import logcat.asLog
 import logcat.logcat
 import retrofit2.HttpException
 
@@ -1105,13 +1108,15 @@ data class PricingPhase(
     val billingPeriod: String,
 
 ) {
-    internal fun getBillingPeriodInDays(): Int? =
-        when (billingPeriod) {
-            "P1W" -> 7
-            "P1M" -> 30
-            "P1Y" -> 365
-            else -> null
+    internal fun getBillingPeriodInDays(): Int? {
+        return try {
+            val period = Period.parse(billingPeriod)
+            return period.days + period.months * 30 + period.years * 365
+        } catch (e: DateTimeParseException) {
+            logcat { "Subs: Failed to parse billing period \"$billingPeriod\": ${e.asLog()}" }
+            null
         }
+    }
 }
 
 data class ValidatedTokenPair(

subscriptions/subscriptions-impl/src/test/java/com/duckduckgo/subscriptions/impl/billing/PricingPhaseTest.kt

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2025 DuckDuckGo
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.duckduckgo.subscriptions.impl.billing
+
+import com.duckduckgo.subscriptions.impl.PricingPhase
+import junit.framework.TestCase.assertEquals
+import junit.framework.TestCase.assertNull
+import org.junit.Test
+
+class PricingPhaseTest {
+    @Test
+    fun billingPeriodParsesDaysCorrectly() {
+        val phase = PricingPhase(formattedPrice = "Free", billingPeriod = "P10D")
+        assertEquals(10, phase.getBillingPeriodInDays())
+    }
+
+    @Test
+    fun billingPeriodParsesWeeksCorrectly() {
+        val phase = PricingPhase(formattedPrice = "Free", billingPeriod = "P2W")
+        assertEquals(14, phase.getBillingPeriodInDays())
+    }
+
+    @Test
+    fun billingPeriodParsesMonthsCorrectly() {
+        val phase = PricingPhase(formattedPrice = "Free", billingPeriod = "P2M")
+        assertEquals(60, phase.getBillingPeriodInDays())
+    }
+
+    @Test
+    fun billingPeriodParsesYearsCorrectly() {
+        val phase = PricingPhase(formattedPrice = "Free", billingPeriod = "P1Y")
+        assertEquals(365, phase.getBillingPeriodInDays())
+    }
+
+    @Test
+    fun billingPeriodParsesMixedPeriodCorrectly() {
+        val phase = PricingPhase(formattedPrice = "Free", billingPeriod = "P1Y2M10D")
+        val expectedDays = 1 * 365 + 2 * 30 + 10 // 365 + 60 + 10 = 435
+        assertEquals(expectedDays, phase.getBillingPeriodInDays())
+    }
+
+    @Test
+    fun billingPeriodEmptyReturnsNull() {
+        val phase = PricingPhase(formattedPrice = "$0", billingPeriod = "")
+        assertNull(phase.getBillingPeriodInDays())
+    }
+
+    @Test
+    fun billingPeriodReturnsNullForInvalidFormat() {
+        val phase = PricingPhase(formattedPrice = "Free", billingPeriod = "INVALID")
+        assertNull(phase.getBillingPeriodInDays())
+    }
+}