Improve least upper bound tests

Description of changes

Update tests for least upper bounds computation in the validator.

The existing tests only tested the permissive mode LUB, I've updated them to also test strict mode.
The tests for action entity LUB were not actually exercising the action entity specific code. They constructed entity-types with the type name Action, but the validator has a specific ActionEntity type that was not tested.
Added an assertion to check that types are subtypes of their least upper bound, uncovering a bug where Action ⊔ Action = Action, but Action was not a subtype of Action. Since the LUB computation was still correct, this does not effect typechecking.

The change in this PR is (choose one, and delete the other options):

[ ] A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).
[ ] A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).
[ ] A bug fix or other functionality change requiring a patch to cedar-policy.
[x] A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)
[ ] A change (breaking or otherwise) that only impacts unreleased or experimental code.

I confirm that this PR (choose one, and delete the other options):

[ ] Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).
[x] Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

[x] Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.
[ ] Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-spec, and how you have tested that your updates are correct.)
[ ] Requires updates, but I do not plan to make them in the near future. (Make sure that your changes are hidden behind a feature flag to mark them as experimental.)
[ ] I'm not sure how my change impacts cedar-spec. (Post your PR anyways, and we'll discuss in the comments.)

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

[x] Does not require updates because my change does not impact the Cedar language specification.
[ ] Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-docs. PRs should be targeted at a staging-X.Y branch, not main.)
[ ] I'm not sure how my change impacts the documentation. (Post your PR anyways, and we'll discuss in the comments.)