fix: ensure session is registered with Hotswapper

[mcollovati]

Description

VaadinSession references are registered in Hotswapper in a session init listener. However, the listener might not always been invoked, for example if the HTTP session is serialized and deserialized on server restart. This change registers a UI init listener to make sure VaadinSession reference is also registered for existing sessions.

Fixes #19973

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• 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.

[github-actions]

Test Results

1 137 files  ± 0  1 137 suites  ±0   1h 9m 58s ⏱️ - 11m 35s
7 399 tests ± 0  7 349 ✅ ± 0  50 💤 ±0  0 ❌ ±0 
7 759 runs  +39  7 699 ✅ +39  60 💤 ±0  0 ❌ ±0 

Results for commit adbfe54. ± Comparison against base commit bf420c4.

♻️ This comment has been updated with latest results.

[sonarcloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[vaadin-bot]

This ticket/PR has been released with Vaadin 24.5.0.beta2 and is also targeting the upcoming stable 24.5.0 version.