fix(explorer): fix profile thread selection

[roaga]

Before we always selected the main thread, but often the main application logic is running in a different thread (e.g. in Seer, it's an AnyIO worker thread). Based on how the frontend determines which thread to show by default, the solution is to just use the thread from the first sample.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/seer/explorer/utils.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #103426      +/-   ##
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- Coverage   80.78%    80.67%   -0.12%     
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  Files        9248      9243       -5     
  Lines      397172    394931    -2241     
  Branches    25158     25158              
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- Hits       320841    318594    -2247     
- Misses      75884     75890       +6     
  Partials      447       447