fix: Do not overwrite baggage header contents if it already exists

[jakubsomonday]

When making outgoing HTTP requests, the SDK was overwriting any pre-existing baggage header on the request with Sentry's own baggage values. This caused loss of third-party baggage entries (e.g., routingKey, tenantId) that were already set on the request.

This fix updates set_propagation_headers to merge Sentry's baggage with any existing baggage on the request instead of replacing it, following the W3C Baggage specification which allows comma-separated list entries from multiple providers.

A similar issue was previously identified and fixed in the Python SDK: getsentry/sentry-python#2191

Changes

• Modified Sentry::Utils::HttpTracing#set_propagation_headers to check for an existing baggage header and merge values instead of overwriting
• Added test cases covering both the merge scenario and the default (no pre-existing baggage) scenario

Fixes #2894

[solnic]

Thank you! I'm gonna merge this in and follow-up with additional features that will ensure we're not exceeding W3C limits with the baggage content 😄

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.48%. Comparing base (7d10c02) to head (f255cc0).
⚠️ Report is 54 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2896      +/-   ##
==========================================
+ Coverage   90.13%   90.48%   +0.34%     
==========================================
  Files         132      141       +9     
  Lines        5301     5600     +299     
==========================================
+ Hits         4778     5067     +289     
- Misses        523      533      +10     

Components	Coverage Δ
sentry-ruby	97.28% <98.31%> (-0.01%)	⬇️
sentry-rails	55.35% <20.00%> (-0.15%)	⬇️
sentry-sidekiq	95.05% <100.00%> (ø)
sentry-resque	94.44% <ø> (ø)
sentry-delayed_job	94.68% <ø> (ø)
sentry-opentelemetry	99.56% <100.00%> (+0.25%)	⬆️

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry/utils/http_tracing.rb	97.43% <100.00%> (+0.21%)	⬆️

... and 1 file with indirect coverage changes

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