fix(entity): re-index ContributionCollection constructor arguments

array_map() over the variadic constructor can produce non-sequential or
string keys, which doesn't match the array<int, Contribution> property
type; re-index with array_values(). Also documents the iterator/count
accessors.

src/Entity/ContributionCollection.php

@@ -14,14 +14,16 @@ final class ContributionCollection implements \IteratorAggregate, \Countable
 
     public function __construct(Contribution ...$contributions)
     {
-        $this->contributions = $contributions;
+        $this->contributions = array_values($contributions);
     }
 
+    /** @return \ArrayIterator<int, Contribution> */
     public function getIterator(): \ArrayIterator
     {
         return new \ArrayIterator($this->contributions);
     }
 
+    /** Number of contributions held in this collection. */
     public function count(): int
     {
         return count($this->contributions);