fix: Always set _spotlight_url

[BYK]

The conditional early exit in SpotlightMiddleware may cause attribute access errors when trying to check if _spotlight_url is set or not. This patch sets it to None explicitly at class level.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.55%. Comparing base (f8ec572) to head (cc69ef1).
Report is 2 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/spotlight.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4186      +/-   ##
==========================================
- Coverage   79.59%   79.55%   -0.05%     
==========================================
  Files         141      141              
  Lines       15736    15737       +1     
  Branches     2675     2675              
==========================================
- Hits        12525    12519       -6     
- Misses       2367     2374       +7     
  Partials      844      844              

Files with missing lines	Coverage Δ
sentry_sdk/spotlight.py	56.75% <33.33%> (+0.39%)	⬆️

... and 2 files with indirect coverage changes

[BYK]

Test fail looks unrelated?