fix: read and migrate v3 session format to v4

[guabu]

📋 Changes

When migrating from v3 to v4 of the SDK, the session format and cookie names differ. This results in the SDK not recognizing that the user has a session and treating them as logged-out.

This PR adds support for reading the v3 session format in both stateless and stateful modes and normalizes them to the v4 format. This ensures that users with existing sessions will be respected in v4.

📎 References

• fixes: Upgrade to v4 causes logout for existing users #1894

🎯 Testing

• Unit tests have been added for both stateful and stateless modes
• Manual tests were run by following the reproduction steps in the referenced issue above to confirm that v3 sessions are smoothly migrated (both stateful and stateless modes)
• Custom cookie names were also tested to ensure they are recognized and migrated

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.43066% with 9 lines in your changes missing coverage. Please review.

Project coverage is 76.06%. Comparing base (33c210d) to head (515d714).

Files with missing lines	Patch %	Lines
src/server/cookies.ts	91.66%	4 Missing ⚠️
src/server/session/stateful-session-store.ts	91.17%	3 Missing ⚠️
src/server/session/normalize-session.ts	95.65%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1923      +/-   ##
==========================================
+ Coverage   74.69%   76.06%   +1.37%     
==========================================
  Files          19       20       +1     
  Lines        1379     1496     +117     
  Branches      210      233      +23     
==========================================
+ Hits         1030     1138     +108     
- Misses        342      351       +9     
  Partials        7        7              

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