ref(pydantic-ai): Drop support for stream_gen_ai_spans

[alexander-alderman-webb]

Description

Set stream_gen_ai_spans=False to maintain assertions for the static trace lifecycle.
Transaction support will be removed as a follow up.

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

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 7398648. Configure here.}

[github-actions]

Codecov Results 📊

✅ 115710 passed | ⏭️ 5544 skipped | Total: 121254 | Pass Rate: 95.43% | Execution Time: 387m 52s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -16441
Passed Tests	📉 -14875
Failed Tests	—
Skipped Tests	📉 -1566

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2265 uncovered lines.
✅ Project coverage is 90.43%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    90.24%    90.43%    +0.19%
==========================================
  Files          193       185        -8
  Lines        25715     23671     -2044
  Branches      9502      8704      -798
==========================================
+ Hits         23206     21406     -1800
- Misses        2509      2265      -244
- Partials      1435      1364       -71

---

Generated by Codecov Action