TW-3961: include grantID in get folders response

[googsvg]

Ticket: TW-3961

Problem

Folder.id and Folder.grantId were declared as non-nullable String with no default value. When the select query parameter was used without including these fields, Moshi
threw:

Required value 'grantId' (JSON name 'grant_id') missing at $.data[1]

Fix

Added empty string defaults to both fields so Moshi falls back gracefully instead of crashing. Type remains String (non-nullable) — no breaking change for existing consumers.

Changes

• Folder.kt — id and grantId default to ""
• FoldersTests.kt — 2 new serialization tests covering partial responses via select

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.47%. Comparing base (ded101e) to head (9cadacf).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #313   +/-   ##
=========================================
  Coverage     82.47%   82.47%           
  Complexity      357      357           
=========================================
  Files            37       37           
  Lines          1027     1027           
  Branches         95       95           
=========================================
  Hits            847      847           
  Misses          121      121           
  Partials         59       59           

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