session: support regenerate ID for exsiting sessions

[unknwon]

Describe the pull request

Link to the issue: https://github.com/sourcegraph/sourcegraph-accounts/issues/193

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.79%. Comparing base (4a24cf9) to head (21ff3b2).

❗ Current head 21ff3b2 differs from pull request most recent head 03ff561. Consider uploading reports for the commit 03ff561 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
+ Coverage   75.23%   75.79%   +0.56%     
==========================================
  Files          10        2       -8     
  Lines         759      157     -602     
==========================================
- Hits          571      119     -452     
+ Misses        107       21      -86     
+ Partials       81       17      -64     

[evict]

Nice! 👍