Fix clearing timer of SessionObserver

[bjoernricks]

What

Fix clearing timer of SessionObserver

Why

Add tests for SessionObserver which discovered a bug in clearing a running timer during unmount.

Checklist

• Tests

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Snapshot Warnings

⚠️: No snapshots were found for the head SHA 9dab77c.

Ensure that dependencies are being submitted on PR branches and consider enabling retry-on-snapshot-warnings. See the documentation for more information and troubleshooting advice.

Scanned Files

None

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.00%. Comparing base (cdfea48) to head (9dab77c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5535      +/-   ##
==========================================
+ Coverage   83.99%   84.00%   +0.01%     
==========================================
  Files        1290     1291       +1     
  Lines       33760    33792      +32     
  Branches    10089    10054      -35     
==========================================
+ Hits        28356    28387      +31     
- Misses       4853     4854       +1     
  Partials      551      551              

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