fix(alias): include distinct_id in $create_alias event properties

.changeset/alias-distinct-id-property.md

@@ -0,0 +1,5 @@
+---
+"posthog": patch
+---
+
+fix: include `distinct_id` alongside `alias` in `$create_alias` event properties

posthog/src/main/java/com/posthog/PostHog.kt

@@ -1319,6 +1319,9 @@ public class PostHog private constructor(
 
         val props = mutableMapOf<String, Any>()
         props["alias"] = alias
+        // The source identity is also duplicated into the properties so consumers that only
+        // read the event properties can resolve both sides of the alias linkage.
+        props["distinct_id"] = distinctId
 
         capture(PostHogEventName.CREATE_ALIAS.event, properties = props)
     }

posthog/src/test/java/com/posthog/PostHogTest.kt

@@ -2008,6 +2008,7 @@ internal class PostHogTest {
         assertEquals("\$create_alias", theEvent.event)
         assertNotNull(theEvent.distinctId)
         assertEquals("theAlias", theEvent.properties!!["alias"] as String)
+        assertEquals(theEvent.distinctId, theEvent.properties!!["distinct_id"] as String)
 
         sut.close()
     }