fix(profiling): start cancels prior stop request

[armcknight]

After some discussion around how stop should work for the upcoming API changes, I realized this was incorrectly coded (or, correctly coded an incorrect assumption).

Stopping the continuous profiler doesn't immediately stop it, rather, it allows the current in-flight chunk to complete, then stops it. If a new call to start comes in, it should "cancel" the pending stop request, and simply allow the same continuous profiler to continue running.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.706%. Comparing base (abff431) to head (7f6648d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #4993       +/-   ##
=============================================
- Coverage   92.707%   92.706%   -0.002%     
=============================================
  Files          672       672               
  Lines        82177     82205       +28     
  Branches     29839     29851       +12     
=============================================
+ Hits         76184     76209       +25     
- Misses        5897      5900        +3     
  Partials        96        96               

Files with missing lines	Coverage Δ
...urces/Sentry/Profiling/SentryContinuousProfiler.mm	91.176% <100.000%> (ø)
...yProfilerTests/SentryContinuousProfilerTests.swift	97.971% <100.000%> (+0.172%)	⬆️

... and 10 files with indirect coverage changes

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