Add unchecked Entities constructors that skip transitive closure

[Harrichael]

Description of changes

Motivation: callers that keep a large pre-loaded entity graph in memory and already maintain its closure themselves can avoid paying to recompute it on every construction or update. This follows the Entities::decode_unchecked precedent from #2387. No changes to cedar-policy-core; the AssumeAlreadyComputed mode already existed there.

Adds Entities::from_entities_unchecked and Entities::upsert_entities_unchecked to the cedar-policy crate. They mirror from_entities and upsert_entities exactly, except that they pass TCComputation::AssumeAlreadyComputed to the core layer instead of ComputeNow, so the transitive closure of the entity hierarchy is neither computed nor checked. The caller must supply a hierarchy that is already acyclic and transitively closed (every ancestor listed in each entity's parents).

Everything else the checked constructors do still applies: duplicate detection, schema conformance checking, and adding Action entities from the schema in from_entities_unchecked. Only transitive closure computation and cycle detection are skipped.

Issue #, if available

Resolves #2378

Checklist for requesting a review

The change in this PR is:

• A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).

I confirm that this PR:

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).

I confirm that cedar-spec:

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

I confirm that docs.cedarpolicy.com:

• Does not require updates because my change does not impact the Cedar language specification.

[github-actions]

Coverage Report

Head Commit: 15a2c884c2ca01d787f67b719b01dd298df49233

Base Commit: ec30badc0fbc331741982553bbec29af6ba262ea

Download the full coverage report.

Coverage of Added or Modified Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 96.30%

Status: PASSED ✅

Details

File	Status	Covered	Coverage	Missed Lines
cedar-policy/src/api.rs	🟢	26/27	96.30%	690

Coverage of All Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 88.40%

Status: PASSED ✅

Details

Package	Status	Covered	Coverage	Base Coverage
cedar-language-server	🟢	4722/5102	92.55%	92.55%
cedar-policy	🟢	4895/5998	81.61%	81.54%
cedar-policy-cli	🟡	1294/1669	77.53%	77.53%
cedar-policy-core	🟢	24675/27839	88.63%	88.63%
cedar-policy-formatter	🟢	914/1088	84.01%	84.01%
cedar-policy-symcc	🟢	6930/7405	93.59%	93.59%
cedar-wasm	🔴	0/28	0.00%	0.00%