Fix session closing sequence

The refactoring in #606 broke the closing sequence for transacted
sessions, as a closed flag was set to true on closing and a method
called in the closing checks the flag.

This commit adds a closing flag to make the closing idempotent and sets
the closed flag to true when the session is closed.

References #606

Author: rabbitmq-ci

3.x/3.5.0-SNAPSHOT/htmlsingle/index.html

@@ -1785,7 +1785,7 @@ <h2 id="further-reading"><a class="anchor" href="#further-reading"></a>Further R
 <div id="footer">
 <div id="footer-text">
 Version 3.5.0-SNAPSHOT<br>
-Last updated 2025-06-25 06:06:54 UTC
+Last updated 2025-06-25 12:57:43 UTC
 </div>
 </div>
 </body>

3.x/snapshot/htmlsingle/index.html

@@ -1785,7 +1785,7 @@ <h2 id="further-reading"><a class="anchor" href="#further-reading"></a>Further R
 <div id="footer">
 <div id="footer-text">
 Version 3.5.0-SNAPSHOT<br>
-Last updated 2025-06-25 06:06:54 UTC
+Last updated 2025-06-25 12:57:43 UTC
 </div>
 </div>
 </body>