fix: fix session serialization for PUSH with websocket

[mcollovati]

Description

When PUSH transport is websocket, the request associated to atmosphere is a stub implementation that always returns false for 'isRequestedSessionIdValid()' preventing PushSessionTracker to serialize the session. In addition, accessing the request from a background thread may throw IllegalStateException if the servlet container is recycling request objects (e.g. Tomcat facade). With this change, the session validity is checked against the SessionListener that is already observing session lifecycle events.

Fixes #119

Type of change

• [x] Bugfix
• [ ] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [x] I have added tests to ensure my change is effective and works as intended.
• [x] New and existing tests are passing locally with my change.
• [ ] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.