adding decode_unchecked API and refactoring validation

[victornicolet]

Description of changes

Adds a new <target type>::decode_unchecked API to protobuf implems, with a refactoring of where the validation occurs to make it more obvious.

This makes a "breaking change" on the experimental protobufs feature by sealing the Protobuf trait (and adding methods). As we stabilize this, I think it's a good idea, in order to allow adding methods in the future without breaking.

Checklist for requesting a review

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

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

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).

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

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

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

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

[Copilot]

Pull request overview

This PR introduces an unchecked protobuf decoding API (decode_unchecked) across protobuf-backed public types, and refactors protobuf decoding so that structural validation is performed in a single, more explicit place (the checked decode path), while allowing callers to opt out of validation for trusted/round-tripped data.

Changes:

• Adds decode_unchecked to the protobuf decoding interface and centralizes validation in the default decode implementation.
• Refactors proto model ↔ core/API conversions to avoid performing validation during TryFrom, enabling truly unchecked decoding.
• Updates protobuf decoding behavior for Entities to optionally skip transitive-closure computation on the unchecked path and adds roundtrip tests for decode_unchecked.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cedar-policy/src/proto/validator.rs	Stops validating during schema model→core conversion to defer validation to the checked decode path.
cedar-policy/src/proto/traits.rs	Introduces TryValidate and refactors Protobuf::decode to validate the decoded value; adds decode_unchecked.
cedar-policy/src/proto/policy.rs	Removes validation from proto→AST conversions for templates/policy sets to support unchecked decoding.
cedar-policy/src/proto/entities.rs	Factors out entities model conversion with configurable transitive-closure mode (and updates docs).
cedar-policy/src/proto/ast.rs	Removes validation from proto→AST entity/expression conversion so checked decoding can own validation.
cedar-policy/src/proto/api.rs	Updates Protobuf impls to provide decode_unchecked, adds special handling for Entities, and adds tests.
cedar-policy/CHANGELOG.md	Notes the behavior change and introduces decode_unchecked in the unreleased changelog entry.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-actions]

Coverage Report

Head Commit: 505d826a755a3e8362411db2223d4e218b6c46c7

Base Commit: b0708054b119e8c9fb49225df70493628ac63923

Download the full coverage report.

Coverage of Added or Modified Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 86.84%

Status: PASSED ✅

Details

File	Status	Covered	Coverage	Missed Lines
cedar-policy/src/proto/api.rs	🟢	19/20	95.00%	136
cedar-policy/src/proto/ast.rs	🟢	2/2	100.00%
cedar-policy/src/proto/entities.rs	🟢	17/17	100.00%
cedar-policy/src/proto/policy.rs	🟢	1/1	100.00%
cedar-policy/src/proto/traits.rs	🟡	26/35	74.29%	196-198, 212, 214-215, 220, 222-223
cedar-policy/src/proto/validator.rs	🟢	1/1	100.00%

Coverage of All Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 87.87%

Status: PASSED ✅

Details

Package	Status	Covered	Coverage	Base Coverage
cedar-language-server	🟢	4722/5102	92.55%	92.55%
cedar-policy	🟢	4530/5644	80.26%	79.93%
cedar-policy-cli	🟡	1230/1621	75.88%	75.88%
cedar-policy-core	🟢	24429/27725	88.11%	88.11%
cedar-policy-formatter	🟢	914/1088	84.01%	84.01%
cedar-policy-symcc	🟢	6777/7274	93.17%	93.17%
cedar-wasm	🔴	0/28	0.00%	0.00%

[github-actions]

Coverage Report

Head Commit: 41f6e5f3032be4ac56f87ad3b9b7a05c4bd986a3

Base Commit: b0708054b119e8c9fb49225df70493628ac63923

Download the full coverage report.

Coverage of Added or Modified Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 86.67%

Status: PASSED ✅

Details

File	Status	Covered	Coverage	Missed Lines
cedar-policy/src/proto/api.rs	🟢	18/19	94.74%	136
cedar-policy/src/proto/ast.rs	🟢	2/2	100.00%
cedar-policy/src/proto/entities.rs	🟢	17/17	100.00%
cedar-policy/src/proto/policy.rs	🟢	1/1	100.00%
cedar-policy/src/proto/traits.rs	🟡	26/35	74.29%	196-198, 212, 214-215, 220, 222-223
cedar-policy/src/proto/validator.rs	🟢	1/1	100.00%

Coverage of All Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 87.90%

Status: PASSED ✅

Details

Package	Status	Covered	Coverage	Base Coverage
cedar-language-server	🟢	4722/5102	92.55%	--
cedar-policy	🟢	4530/5644	80.26%	--
cedar-policy-cli	🟡	1257/1649	76.23%	--
cedar-policy-core	🟢	24576/27882	88.14%	--
cedar-policy-formatter	🟢	914/1088	84.01%	--
cedar-policy-symcc	🟢	6777/7274	93.17%	--
cedar-wasm	🔴	0/28	0.00%	--