Add tests for starred/watchers/watched activity APIs

[arnavk23]

• Added robust activity coverage for stargazers, starred, watchers, and watched: type checks, non-empty stargazers, count check against subscribers_count when available, and safe handling for users with zero starred/watched repos read_only_api_tests.jl:209-238.
• Tests: Pkg.test(; test_args=["read_only_api_tests"]) passes with your token (only the pre-existing broken case remains).
• Fixes TODO: Fix tests for starred, watchers, and watched #237

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.96%. Comparing base (97e94e4) to head (a88991c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
+ Coverage   58.45%   58.96%   +0.50%     
==========================================
  Files          37       37              
  Lines         982      982              
==========================================
+ Hits          574      579       +5     
+ Misses        408      403       -5     

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[arnavk23]

@DilumAluthge All tests pass. Please review this pr. Thanks!

[DilumAluthge]

Thanks for working on this!

I've left a few minor comments. Let me know if you have any questions.

[arnavk23]

@DilumAluthge please review this pr again. Thanks!

[DilumAluthge]

Thanks for the bump! I've added this to my queue to review, but I might not get to it immediately.