PYTHON-4020 Drivers should unpin connections when ending a session (#2239)

Author: blink1073

test/load_balancer/transactions.json

@@ -1616,6 +1616,50 @@
           ]
         }
       ]
+    },
+    {
+      "description": "pinned connection is released when session ended",
+      "operations": [
+        {
+          "name": "startTransaction",
+          "object": "session0"
+        },
+        {
+          "name": "insertOne",
+          "object": "collection0",
+          "arguments": {
+            "document": {
+              "x": 1
+            },
+            "session": "session0"
+          }
+        },
+        {
+          "name": "commitTransaction",
+          "object": "session0"
+        },
+        {
+          "name": "endSession",
+          "object": "session0"
+        }
+      ],
+      "expectEvents": [
+        {
+          "client": "client0",
+          "eventType": "cmap",
+          "events": [
+            {
+              "connectionReadyEvent": {}
+            },
+            {
+              "connectionCheckedOutEvent": {}
+            },
+            {
+              "connectionCheckedInEvent": {}
+            }
+          ]
+        }
+      ]
     }
   ]
 }