fix(session): prevent accidental deletion of legacy-named session cookie

[nandan-bhat]

When migrating from nextjs-auth0 v3 to v4, developers may want to preserve the legacy session cookie name (appSession). However, in stateful mode, the SDK deletes the legacy cookie unconditionally after setting the new session cookie which leads to the newly set cookie being immediately removed if the names match.

This behavior breaks login flows and causes unexpected session loss.

🛠️ Changes

This PR adds a conditional guard in StatefulSessionStore.set() to ensure that:

if (
    this.sessionCookieName !== LEGACY_COOKIE_NAME &&
    reqCookies.has(LEGACY_COOKIE_NAME)
) {
    resCookies.delete(LEGACY_COOKIE_NAME);
}

This prevents accidental deletion when session.name is set to appSession a common case during v3 to v4 upgrades.

Reported Issue: #2111

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.48%. Comparing base (7bd8398) to head (298a7fc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2114      +/-   ##
==========================================
+ Coverage   82.46%   82.48%   +0.02%     
==========================================
  Files          21       21              
  Lines        2024     2027       +3     
  Branches      355      356       +1     
==========================================
+ Hits         1669     1672       +3     
  Misses        348      348              
  Partials        7        7              

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