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