feat(DRIVERS-2647): ensure endSessions unpins (#1711)

Author: durran

source/load-balancers/tests/transactions.json

@@ -596,3 +596,21 @@ tests:
           - connectionCheckedOutEvent: {}
           # Events for abortTransaction.
           - connectionCheckedInEvent: {}
+
+  - description: pinned connection is released when session ended
+    operations:
+      - *startTransaction
+      - *transactionalInsert
+      - *commitTransaction
+      - &endSession
+        name: endSession
+        object: *session0
+    expectEvents:
+      - client: *client0
+        eventType: cmap
+        events:
+          # Events for the insert and commitTransaction.
+          - connectionReadyEvent: {}
+          - connectionCheckedOutEvent: {}
+          # Events for endSession.
+          - connectionCheckedInEvent: {}

source/load-balancers/tests/transactions.yml

@@ -641,6 +641,7 @@ Drivers MUST unpin a ClientSession in the following situations:
     be unpinned before performing server selection for the first operation of the new transaction.
 5. A non-transactional operation is performed using the ClientSession. The session MUST be unpinned before performing
     server selection for the operation.
+6. The ClientSession is ended either explicitly via `endSession` method, or implicitly when supported by the driver.
 
 Note that committing a transaction on a pinned ClientSession MUST NOT unpin the session as `commitTransaction` may be
 called multiple times.
@@ -1073,6 +1074,8 @@ objective of avoiding duplicate commits.
 
 ## **Changelog**
 
+- 2024-11-01: Specify that ClientSession must be unpinned when ended.
+
 - 2024-10-31: Clarify when drivers must add TransientTransactionError label.
 
 - 2024-10-28: Note read preference must always be primary in a transaction.