fix(server): close SQLite stores in policy tests

[kjlubick]

Summary

I noticed some flaky tests on Windows and I think this is the reason. This explicitly closes the SQLite-backed test store in four policy tests that intermittently triggered a Windows fail-fast exit (0xc0000409) after the tests had already reported success.

Awaiting store closure allows SQLx's SQLite worker threads and keepalive connection to finish shutting down before the test process exits. This only changes test teardown; it does not change policy behavior.

Related Issue

No issue required: small test fix

Changes

• Await SQLite store shutdown in the four policy tests observed failing during Windows ARM64 CI.
• Document why explicit asynchronous cleanup is required.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

The native Windows ARM64 workspace test phase passed three consecutive times.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[github-actions]

All contributors have signed the DCO ✍️ ✅
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[kjlubick]

I have read the DCO document and I hereby sign the DCO.

[pimlock]

/ok to test 29e7c90