bugfix: session write not happening for pages router with chunked cookies

[tusharpandey13]

When using getAccessToken(req, res) or updateSession(req, res, session) in Pages Router with chunked sessions, only the last cookie chunk was being persisted. This caused invalid_grant errors on subsequent requests because the session was corrupted.

Fixes: #2394

Root Cause

Headers.entries() yields each Set-Cookie header separately, but res.setHeader() overwrites previous values instead of appending in saveToSession.

Changes

• client.ts: Updated logic to use pagesRouterRes.appendHeader for set-cookie header.
• added a UT for the same.

Reproduction steps

• Set Access token lifetime to 10s in auth0 dashboard
• Added a pages router page that fetches Access token
• Page refreshed after 10s multiple times
• On second refresh, an invalid_grant error was thrown.

Reproduction failed after fixes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.99%. Comparing base (6690372) to head (989bfdb).

Files with missing lines	Patch %	Lines
src/server/client.ts	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2447      +/-   ##
==========================================
+ Coverage   90.56%   90.99%   +0.43%     
==========================================
  Files          39       39              
  Lines        4515     4520       +5     
  Branches      933      939       +6     
==========================================
+ Hits         4089     4113      +24     
+ Misses        420      401      -19     
  Partials        6        6              

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