fix(spotlight): align behavior with SDK spec

[BYK]

Align the Spotlight implementation with the official spec at
https://develop.sentry.dev/sdk/expected-features/spotlight/

Changes:

1. Fix precedence rules:

   • When spotlight=True and SENTRY_SPOTLIGHT env var is a URL, use the
     env var URL (per spec requirement)
   • Log warning when config URL overrides env var URL
   • Log warning when spotlight=False explicitly disables despite env
     var being set

2. Route all envelopes to Spotlight:

   • Sessions, logs, and metrics now also get sent to Spotlight via the
     _capture_envelope callback
   • Move Spotlight initialization before batchers are created

3. Add exponential backoff retry logic:

   • SpotlightClient now implements proper exponential backoff when the
     Spotlight server is unreachable
   • Skips sending during backoff period to avoid blocking
   • Logs errors only once per backoff cycle

4. Update tests:

   • Fix test expectation for spotlight=True + env URL case
   • Add tests for warning scenarios
   • Add test for session envelope routing to Spotlight

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.93%. Comparing base (5fc28a1) to head (3703b3c).
⚠️ Report is 10 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/spotlight.py	84.78%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5169      +/-   ##
==========================================
+ Coverage   83.89%   83.93%   +0.03%     
==========================================
  Files         181      181              
  Lines       18343    18369      +26     
  Branches     3261     3269       +8     
==========================================
+ Hits        15389    15418      +29     
+ Misses       1946     1944       -2     
+ Partials     1008     1007       -1     

Files with missing lines	Coverage Δ
sentry_sdk/client.py	83.58% <100.00%> (-0.13%)	⬇️
sentry_sdk/spotlight.py	67.36% <84.78%> (+9.70%)	⬆️

... and 3 files with indirect coverage changes

[sentrivana]

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