fix: Work around compromised transitive dependency

[sentrivana]

• mistralai might have been compromised. It's been quarantined by PyPI, so it can't be installed at all.
• The base Pydantic AI package pydantic-ai installs pydantic-ai-slim with a bunch of extras for all sort of AI providers, including mistralai.
• Our test suite fails because we attempt to install pydantic-ai, which then tries to pull in mistralai transitively.

Even though there's now a new release of pydantic-ai without the dependency, older versions still have it, and we still want to test against a variety of versions in our test suite.

The fix: don't install pydantic-ai, but instead install pydantic-ai-slim directly, at least for now until mistralai is restored.

[github-actions]

Codecov Results 📊

✅ 146 passed | Total: 146 | Pass Rate: 100% | Execution Time: 21.68s

📊 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 14343 uncovered lines.
✅ Project coverage is 34.96%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    34.96%    34.96%        —%
==========================================
  Files          190       190         —
  Lines        22052     22052         —
  Branches      7408      7408         —
==========================================
+ Hits          7709      7709         —
- Misses       14343     14343         —
- Partials       807       807         —

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