fix(sampling): isolate manual sampling test trace contexts

[bm1549]

Motivation

Trace ID overlap report (AppGate required)

The manual-sampling tests used the same upstream trace and parent IDs for the keep and drop cases. When uWSGI handled the two requests at once, dd-trace-py correctly grouped their spans into one trace, and the drop request had no serialized sampling priority.

Changes

The upstream-context helper now offsets its fixed IDs by the upstream sampling priority. Each case stays stable for lineage assertions, while keep and drop use separate trace and parent IDs.

Tests

• Temporary event/barrier reproduction against Python 4.14.0, uwsgi-poc, and Agent 7.83.1: 5/5 expected failures with shared IDs; 5/5 passes with isolated IDs
• Explicit mutation check with the same reproduction: failed after removing the ID offset, then passed after restoring it
• TEST_LIBRARY=python ./run.sh SAMPLING tests/test_sampling_manual.py::Test_Manual_Sampling: 2 passed
• ./run.sh TEST_THE_TEST tests/test_the_test/test_sampling_manual.py: 1 passed
• ./format.sh --check: passed

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Reviewer checklist

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[github-actions]

CODEOWNERS have been resolved as:

tests/test_sampling_manual.py                                           @DataDog/system-tests-core

[datadog-datadog-prod-us1-2]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-11T01:59:06.023987Z	1579c77	Draft marked ready

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