Fix type of sample_rate in DSC (and add explanatory tests)

[antonpirker]

In the DSC send in the envelope header for envelopes containing errors the type of sample_rate was float instead of the correct str type.

Fixes #3601

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.34%. Comparing base (5080c76) to head (d43c9c2).
Report is 2 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3603   +/-   ##
=======================================
  Coverage   84.34%   84.34%           
=======================================
  Files         133      133           
  Lines       13929    13929           
  Branches     2938     2938           
=======================================
+ Hits        11748    11749    +1     
+ Misses       1443     1442    -1     
  Partials      738      738           

Files with missing lines	Coverage Δ
sentry_sdk/tracing_utils.py	86.08% <100.00%> (+0.57%)	⬆️

... and 1 file with indirect coverage changes

[sentrivana]

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