Reproduction for sentry-php#1999

[cleptric]

Summary

This PR adds a reproduction for getsentry/sentry-php#1999

The reproduction demonstrates that the SDK incorrectly uses the sentry-sample_rate value from the baggage header to make a sampling decision when the sentry-trace header is missing.

How to Run

cd sentry-php/1999
composer install
php reproduce.php

Output

=== Reproduction for sentry-php#1999 ===

Configuration:
  traces_sample_rate: 0.0 (0% - should never sample)

Incoming Headers:
  sentry-trace: (not present)
  baggage: sentry-sample_rate=1.0,sentry-sampled=true,...

Result:
  Transaction sampled: YES

BUG CONFIRMED: Transaction was sampled despite traces_sample_rate=0.0
The SDK incorrectly used the baggage sample_rate (1.0) instead of the local config.

Expected Behavior

Without a sentry-trace header, the SDK should treat this as a new trace head and use the locally configured traces_sample_rate (0.0) instead of the baggage sample rate (1.0).

[cursor]

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