fix: ListThreadsQueryParams.inFolder now correctly uses single folder ID

[AaronDDM]

What did you do?

The Nylas API only supports filtering by a single folder ID, but the SDK
was incorrectly accepting a List and sending all items to the API.
The API would only use the last item, causing unexpected behavior.

Changes:

• Modified ListThreadsQueryParams.convertToMap() to use only the first
  folder ID from a list, matching API behavior
• Added new inFolder(String) method in Builder (recommended approach)
• Deprecated inFolder(List) method with clear migration guidance
• Added comprehensive test coverage for all scenarios
• Updated documentation with API limitations and usage examples
• Maintained full backward compatibility

The fix ensures proper API behavior while providing a clear migration
path for users. In the next major version, the parameter will be
changed to accept only a String.

Fixes issue where only the last folder ID was used when multiple
folder IDs were provided in the inFolder parameter.

📋 Usage Examples

New recommended approach:

val queryParams = ListThreadsQueryParams.Builder()
    .inFolder("folder-id-123")
    .limit(10)
    .build()

Deprecated but still working:

val queryParams = ListThreadsQueryParams.Builder()
    .inFolder(listOf("folder1", "folder2"))  // Only "folder1" will be used
    .limit(10)
    .build()

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 80.57%. Comparing base (7406677) to head (c9897db).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #280   +/-   ##
=========================================
  Coverage     80.57%   80.57%           
  Complexity      333      333           
=========================================
  Files            36       36           
  Lines          1014     1014           
  Branches         88       88           
=========================================
  Hits            817      817           
  Misses          138      138           
  Partials         59       59           

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