test: Close client on teardown

[alexander-alderman-webb]

Description

Resolve Mistral test flake involving import race by closing old clients (preventing dangling threads).

Issues

Closes #7556

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 📊

✅ 129634 passed | ⏭️ 7148 skipped | Total: 136782 | Pass Rate: 94.77% | Execution Time: 451m 50s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +256
Passed Tests	📈 +256
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2550 uncovered lines.
❌ Project coverage is 90.19%. Comparing base (b454d7d) to head (6ec87ee).

Coverage diff

@@            Coverage Diff             @@
##        master       #PR       +/-##
==========================================
- Coverage    90.20%    90.19%    -0.01%
==========================================
  Files          194       194         —
  Lines        25888     25994      +106
  Branches      9584      9660       +76
==========================================
+ Hits         23352     23444       +92
- Misses        2536      2550       +14
- Partials      1455      1451        -4

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