Fix potential race condition in rule type engine cache

[dmjb]

This addresses a potential race condition I discovered while working on another task. If a rule instance with a new rule type is added to a profile after the rule type cache is populated, but before the profiles and rule instances are queried, the evaluation will fail because the new rule type will be missing from the cache.

This changes the logic to query for rule types which are not found in the cache, and create a new rule type engine instance and cache it if we get a result.

Alternatives Considered

I could simplify this to remove the pre-population and query the database for each rule type when we encounter it first. This would simplify the logic, but at the expense of throwing away the performance gains from loading most of the rules we need in one query.

Summary

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Fixes #(related issue)

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• Bug fix (resolves an issue without affecting existing features)
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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
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[coveralls]

coverage: 54.393% (+0.08%) from 54.311%
when pulling 163b9d3 on rule-cache-race-condition
into e0b91af on main.