fix(auth): Resolve logout issue caused by JWT token serialization breaking change

[tylerjroach]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:
#3159

Description of changes:

Fix: Resolve logout issue caused by JWT token serialization breaking change

Problem

Commit 047483866231622a362f736350f600072affad86 introduced type-safe JWT token classes but inadvertently created a breaking change in the serialized data structure. This caused existing users to be automatically logged out when updating to the new version.

Root Cause: The new token classes (IdToken, AccessToken, RefreshToken) were serialized as nested objects instead of flat strings, causing deserialization failures for existing stored tokens.

Impact

• Existing users: Tokens stored as "idToken": "eyJhbGc..." could not be deserialized
• Updated users: Some users got tokens stored as "idToken": {"tokenValue": "eyJhbGc..."}
• Result: Automatic logout on app launch after update

Solution

Added backward-compatible serialization that:

1. Maintains flat format: Tokens serialize as raw strings ("idToken": "eyJhbGc...")
2. Handles all formats: Deserializes both flat strings and nested objects seamlessly
3. Auto-migration: Nested tokens are automatically flattened on next save
4. Type safety: Preserves all benefits of the new JWT parsing classes

Changes Made

Core Implementation

• Custom serializers: Added IdTokenAsStringSerializer, AccessTokenAsStringSerializer, RefreshTokenAsStringSerializer
• Format detection: extractTokenValue() helper detects and handles both formats

Comprehensive Testing

• Flat format compatibility: Existing tokens deserialize correctly
• Nested format support: Problematic tokens from broken version work
• Auto-migration: Nested tokens convert to flat format on serialization
• Format consistency: All paths produce identical results

Backward Compatibility Matrix

User Type	Stored Format	Result
Existing users	"idToken": "eyJhbGc..."	✅ Works seamlessly
Updated users	"idToken": {"tokenValue": "eyJhbGc..."}	✅ Auto-migrated to flat
New users	"idToken": "eyJhbGc..."	✅ Clean flat format

Files Changed

• Tokens.kt: Added backward-compatible serializers
• TokensTest.kt: Comprehensive serialization compatibility tests

Testing

• ✅ All existing tests pass
• ✅ New tests cover flat format, nested format, and migration scenarios
• ✅ JWT parsing functionality preserved across all formats

This fix ensures no users experience logout issues while maintaining all the benefits of the type-safe JWT implementation.

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(storage): message, feat(auth): message, chore(all): message)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❌ Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.78%. Comparing base (e54f435) to head (b431c24).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3162      +/-   ##
==========================================
+ Coverage   54.77%   54.78%   +0.01%     
==========================================
  Files        1047     1047              
  Lines       31251    31267      +16     
  Branches     4673     4674       +1     
==========================================
+ Hits        17119    17131      +12     
- Misses      12324    12328       +4     
  Partials     1808     1808              

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[harsh62]

LGTM