Add missing support for legacy chunked cookies

[tusharpandey13]

Fixes: #2066 (Chunked v3 session cookies lost after v4 upgrade)

Changes

• Updated cookie logic to recognize both current (__index) and legacy (.index) cookie name formats.
• Added unit test for the same.

Testing

PASSING

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.51%. Comparing base (3ce7c59) to head (ba9c5be).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2071      +/-   ##
==========================================
+ Coverage   82.25%   82.51%   +0.26%     
==========================================
  Files          21       21              
  Lines        1989     2019      +30     
  Branches      348      352       +4     
==========================================
+ Hits         1636     1666      +30     
  Misses        347      347              
  Partials        6        6              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

these changes have been done