test(pydantic-ai): Remove vacuous _set_model_data() tests

[alexander-alderman-webb]

Description

The assertions are trivial because a manual transaction is created, and we should not test private functions to begin with.

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 📊

✅ 131835 passed | ⏭️ 7131 skipped | Total: 138966 | Pass Rate: 94.87% | Execution Time: 471m 29s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -40
Passed Tests	📉 -40
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2532 uncovered lines.
❌ Project coverage is 90.18%. Comparing base (48c48d2) to head (1fc103d).

Coverage diff

@@            Coverage Diff             @@
##        master       #PR       +/-##
==========================================
- Coverage    90.20%    90.18%    -0.02%
==========================================
  Files          193       193         —
  Lines        25788     25788         —
  Branches      9530      9530         —
==========================================
+ Hits         23261     23256        -5
- Misses        2527      2532        +5
- Partials      1432      1430        -2

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