remove LiteralPolicy for proto and merge validation steps

[victornicolet]

Description of changes

This part 2 of #2469:

• removes the LiteralPolicySet struct that was used as an intermediate between models::PolicySet and ast::PolicySet.
• merges the validation steps for PolicySet that were copy-pasted in the previous PR,
• merges the validation steps for Policy that were separated by LiteralPolicy.

The Policy invariants on slot well formedness is now entirely in the Template to Policy conversion (try_as_policy enforces the same as link of ast::Template).

Issue #, if available

Checklist for requesting a review

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.
• 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).
• 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):

• 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.)
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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.
• 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.)

[victornicolet]

This is similar to what LiteralPolicy::reify(self,... was doing.

[victornicolet]

The test mostly change the input type and tests reify as opposed to try_from.

[Copilot]

Pull request overview

This PR completes the protobuf validation/refactor work started in #2469 by removing the LiteralPolicy / LiteralPolicySet intermediate representation and consolidating policy/policyset validation and reification directly into ast::Policy/ast::PolicySet conversion paths.

Changes:

• Introduces a reify(models::Policy, templates) conversion flow to produce ast::Policy directly (removing ast::LiteralPolicy usage in protobuf code).
• Refactors protobuf models::PolicySet -> ast::PolicySet conversion to build template/link maps in one pass with duplicate detection.
• Removes LiteralPolicy (and associated reification error type) from cedar-policy-core, replacing static-policy creation with Template::try_as_policy.

Reviewed changes

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

File	Description
cedar-policy/src/proto/policy.rs	Switches protobuf policy conversion to direct ast::Policy reification and refactors PolicySet conversion + tests accordingly.
cedar-policy-core/src/ast/policy.rs	Adds Template::try_as_policy and removes the LiteralPolicy IR and related reification logic/types.

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[github-actions]

Coverage Report

Head Commit: b520054e82f03f8b6c9a7c6c19a2455fa8fc25e4

Base Commit: e75a13b731928f70e80e75a859fb97fc47987737

Download the full coverage report.

Coverage of Added or Modified Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 92.86%

Status: PASSED ✅

Details

File	Status	Covered	Coverage	Missed Lines
cedar-policy-core/src/ast/policy.rs	🔴	4/9	44.44%	723-727
cedar-policy/src/proto/policy.rs	🟢	61/61	100.00%

Coverage of All Lines of Rust Code

Required coverage: 80.00%

Actual coverage: 88.27%

Status: PASSED ✅

Details

Package	Status	Covered	Coverage	Base Coverage
cedar-language-server	🟢	4722/5102	92.55%	--
cedar-policy	🟢	4635/5750	80.61%	--
cedar-policy-cli	🟡	1286/1675	76.78%	--
cedar-policy-core	🟢	24670/27826	88.66%	--
cedar-policy-formatter	🟢	914/1088	84.01%	--
cedar-policy-symcc	🟢	6833/7313	93.44%	--
cedar-wasm	🔴	0/28	0.00%	--