fix(sampling): sentry-trace should take precedence over baggage

[sentrivana]

Description

We were making the wrong sampling decision in a specific case: when the incoming sentry-trace ended in -0, but the incoming baggage had a combination of sentry-sample_rate and sentry-sample_rand that would've actually produced a positive sampling decision, we would prioritize the information from the baggage over the parent sampling decision from sentry-trace.

This PR aligns the behavior with the legacy codepath, where we'd set parent_sampled as the new sample_rate.

Issues

• closes https://linear.app/getsentry/issue/PY-2618/incorrect-sampling-decision-propagated-in-some-cases-in-span-streaming
• closes Incorrect sampling decision propagated in some cases in span streaming #6856

Reminders

• Please add tests to validate your changes, and lint your code using uv run ruff.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[linear-code]

PY-2618

[github-actions]

Codecov Results 📊

✅ 95930 passed | ⏭️ 6340 skipped | Total: 102270 | Pass Rate: 93.8% | Execution Time: 336m 27s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +415
Passed Tests	📈 +377
Failed Tests	—
Skipped Tests	📈 +38

All tests are passing successfully.

✅ Patch coverage is 83.33%. Project has 2512 uncovered lines.
✅ Project coverage is 89.67%. Comparing base (base) to head (head).

Files with missing lines (1)

File	Patch %	Lines
sentry_sdk/tracing_utils.py	83.33%	⚠️ 1 Missing and 1 partials

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.65%    89.67%    +0.02%
==========================================
  Files          193       193         —
  Lines        24324     24321        -3
  Branches      8572      8572         —
==========================================
+ Hits         21807     21809        +2
- Misses        2517      2512        -5
- Partials      1395      1397        +2

---

Generated by Codecov Action

[ericapisani]

LGTM, just noticed a small thing

[ericapisani]

Why not add these to the initial definition of incoming_baggage above?

[sentrivana]

Thanks for noticing! Copied this test from another one that only did this conditionally and missed this.

Fixed in ddc7386