Fix GET user neighbors APIs

[zhenghaoz]

No description provided.

[Copilot]

Pull request overview

This PR fixes the GET user neighbors APIs by adding a new endpoint for user-to-user recommendations and fixing a bug in the existing user neighbors endpoint. The main issue being addressed is the incorrect category parameter being passed to the search function, which was filtering results incorrectly.

• Adds new /user-to-user/{name}/{user-id} endpoint that aligns with the item-to-item pattern
• Fixes bug in /user/{user-id}/neighbors endpoint where []string{""} was incorrectly passed as categories instead of nil
• Adds test coverage for both the new endpoint and the fixed endpoint

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
server/rest.go	Adds new getUserToUser handler and fixes category parameter bug in getUserNeighbors by passing nil instead of []string{""}
server/rest_test.go	Adds TestUserToUser to verify both /user-to-user/{name}/{user-id} and /user/{user-id}/neighbors endpoints return expected results

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.14%. Comparing base (3b8ecdd) to head (7d65335).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1110      +/-   ##
==========================================
+ Coverage   71.09%   71.14%   +0.05%     
==========================================
  Files          86       86              
  Lines       15066    15071       +5     
==========================================
+ Hits        10711    10723      +12     
+ Misses       3299     3291       -8     
- Partials     1056     1057       +1     

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