ref(openai-agents): Stop setting gen_ai.response.model on Invoke Agent spans

[alexander-alderman-webb]

Description

The attribute is ambiguous as an agent can use multiple models in the course of its execution.

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 📊

✅ 99102 passed | ⏭️ 6479 skipped | Total: 105581 | Pass Rate: 93.86% | Execution Time: 356m 11s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ No test changes detected

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2506 uncovered lines.
✅ Project coverage is 89.96%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.93%    89.96%    +0.03%
==========================================
  Files          193       193         —
  Lines        24963     24954        -9
  Branches      9010      9004        -6
==========================================
+ Hits         22448     22448         —
- Misses        2515      2506        -9
- Partials      1436      1434        -2

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