test: Add streaming tests to test_http_headers

[sentrivana]

Description

Add span streaming variants to tests in test_http_headers

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 📊

✅ 91914 passed | ⏭️ 6302 skipped | Total: 98216 | Pass Rate: 93.58% | Execution Time: 314m 10s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +60
Passed Tests	📈 +60
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2477 uncovered lines.
✅ Project coverage is 89.68%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.68%    89.68%        —%
==========================================
  Files          193       193         —
  Lines        24004     24004         —
  Branches      8340      8340         —
==========================================
+ Hits         21528     21527        -1
- Misses        2476      2477        +1
- Partials      1385      1386        +1

---

Generated by Codecov Action