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]

🙏🏻