test: Add streaming tests to test_sampling

[sentrivana]

Description

Only ported tests that also make sense in streaming mode. For example, tests testing explicit sampled decisions as args to start_transaction were not ported.

Issues

Part of #5395

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

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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[alexander-alderman-webb]

🚀

[github-actions]

Codecov Results 📊

✅ 90555 passed | ⏭️ 6302 skipped | Total: 96857 | Pass Rate: 93.49% | Execution Time: 313m 8s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +450
Passed Tests	📈 +450
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2444 uncovered lines.
✅ Project coverage is 89.74%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.74%    89.74%        —%
==========================================
  Files          192       192         —
  Lines        23823     23823         —
  Branches      8226      8226         —
==========================================
+ Hits         21378     21379        +1
- Misses        2445      2444        -1
- Partials      1349      1348        -1

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