ref(openai-agents): Remove handoff spans

[alexander-alderman-webb]

Description

Remove handoff spans since the product does not support them.

Remove test_handoff_span() and test_max_turns_before_handoff_span() since they only have assertions on handoff spans.

Issues

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 📊

✅ 65988 passed | ❌ 1 failed | ⏭️ 2999 skipped | Total: 68988 | Pass Rate: 95.65% | Execution Time: 191m 10s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -67538
Passed Tests	📉 -63390
Failed Tests	📈 +1
Skipped Tests	📉 -4149

✅ Patch coverage is 100.00%. Project has 2075 uncovered lines.
❌ Project coverage is 90.06%. Comparing base (9879c76) to head (40c7270).

Coverage diff

@@            Coverage Diff             @@
##        master       #PR       +/-##
==========================================
- Coverage    90.20%    90.06%    -0.14%
==========================================
  Files          194       177       -17
  Lines        25888     20878     -5010
  Branches      9584      7512     -2072
==========================================
+ Hits         23352     18803     -4549
- Misses        2536      2075      -461
- Partials      1455      1175      -280

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