test: Add streaming propagation tests

[sentrivana]

Description

Some tests in this file were testing the unsupported case of using a top-level start_span (instead of start_transaction) and checking that the resulting span then doesn't produce propagation headers. As this case now has no counterpart in span streaming, I didn't port those tests.

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

[github-actions]

Codecov Results 📊

✅ 90105 passed | ⏭️ 6302 skipped | Total: 96407 | Pass Rate: 93.46% | Execution Time: 313m 21s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +30
Passed Tests	📈 +30
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2437 uncovered lines.
✅ Project coverage is 89.77%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.77%    89.77%        —%
==========================================
  Files          192       192         —
  Lines        23823     23823         —
  Branches      8226      8226         —
==========================================
+ Hits         21386     21386         —
- Misses        2437      2437         —
- Partials      1348      1348         —

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