ref(langgraph): 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

[github-actions]

Codecov Results 📊

✅ 116884 passed | ⏭️ 5544 skipped | Total: 122428 | Pass Rate: 95.47% | Execution Time: 380m 3s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -15267
Passed Tests	📉 -13701
Failed Tests	—
Skipped Tests	📉 -1566

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2262 uncovered lines.
✅ Project coverage is 90.44%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    90.24%    90.44%     +0.2%
==========================================
  Files          193       185        -8
  Lines        25715     23671     -2044
  Branches      9502      8704      -798
==========================================
+ Hits         23206     21409     -1797
- Misses        2509      2262      -247
- Partials      1435      1363       -72

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