fix: delete v3 appSession cookie on logout

[gmurphey]

Summary

• When upgrading from v3 to v4, the legacy appSession cookie was only cleaned up during login (set()), not during logout (delete()). This caused users with a lingering v3 cookie to bypass authentication on their next login attempt.
• Adds appSession cookie deletion to StatelessSessionStore.delete() and StatefulSessionStore.delete(), mirroring the existing cleanup logic in their set() methods.

Test plan

• Added test: stateless delete() removes legacy chunked appSession cookie when present
• Added test: stateful delete() removes legacy appSession cookie when present
• Added test: stateful delete() skips redundant deletion when session cookie name is already appSession
• All 891 existing tests continue to pass

🤖 Generated with Claude Code

[Piyush-85]

@gmurphey Thanks for the contribution. This fix was critical and LGTM.
Approving and merging this.

Note - Will add a section Session Cookie Migration in the V4 Migration Guide.md to reflect this change.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.85%. Comparing base (444e2b3) to head (53315ac).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2552      +/-   ##
==========================================
+ Coverage   89.83%   89.85%   +0.01%     
==========================================
  Files          67       67              
  Lines        8119     8135      +16     
  Branches     1739     1743       +4     
==========================================
+ Hits         7294     7310      +16     
  Misses        813      813              
  Partials       12       12              

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