fix(go): handle proto direct assignments in DSL transform

[ErlingWis]

Description

Fixes TransformJSONProtoToDSL for in-memory authorization models produced by TransformModuleFilesToModel when relations contain direct
assignments.

What problem is being solved?

Direct assignments are represented by the Userset_This protobuf oneof case. The previous implementation detected direct assignments with
GetThis() != nil, which can fail when the oneof case is set but the inner DirectUserset sentinel is nil.

This caused valid module models to fail when round-tripping through:

TransformModuleFilesToModel -> TransformJSONProtoToDSL

How is it being solved?

TransformJSONProtoToDSL now detects direct assignments by checking whether the userset is the Userset_This oneof case, instead of
requiring GetThis() to return a populated DirectUserset.

The DSL-to-proto transformer also now populates DirectUserset when creating direct-assignment rewrites.

What changes are made to solve it?

• Detect direct assignments by checking the Userset_This oneof case.
• Populate DirectUserset when transforming DSL direct assignments to proto.
• Add module round-trip test coverage for TransformJSONProtoToDSL, with and without source information.

References

closes #620

Review Checklist

• [x ] I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• [ x] I have added tests to validate that the change in functionality is working as expected

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of direct assignments when converting between JSON/protobuf and DSL, making rewritten output more consistent and reliable.
  • Fixed direct-assignment detection in nested relation cases so conversion results are handled correctly across different rewrite paths.

• Tests

  • Added coverage for JSON-to-DSL conversion, including output with and without source information.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ErlingWis / name: Erling Wisløff (7f33cfc)

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Walkthrough

Direct-assignment usersets are now emitted with an explicit DirectUserset payload, and the JSON-to-DSL transformer recognizes direct assignments by concrete protobuf type across validation, parsing, and prioritization. A new test covers module-to-DSL transforms with and without source information.

Changes

Direct assignment round-trip

Layer / File(s)	Summary
Emit and detect direct assignment pkg/go/transformer/dsltojson.go, pkg/go/transformer/jsontodsl.go	TransformJSONProtoToDSL now emits direct assignments with an explicit DirectUserset, and JSON-to-DSL direct-assignment handling uses concrete Userset_This checks in validation, parsing, and prioritization.
Module-to-DSL coverage pkg/go/transformer/jsontodsl_test.go	A new test transforms module cases to a model, runs JSON-to-DSL with and without source information, and compares both DSL outputs to expected fixtures.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately summarizes the main change: handling proto direct assignments in DSL transforms.
Linked Issues check	✅ Passed	The changes address #620 by fixing direct-assignment detection and adding round-trip coverage for TransformJSONProtoToDSL.
Out of Scope Changes check	✅ Passed	The added test coverage and transformer adjustments stay within the direct-assignment DSL transform fix scope.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Copilot]

Pull request overview

This PR fixes the Go TransformJSONProtoToDSL transformer to correctly recognize direct assignments when the protobuf Userset oneof is set to Userset_This but the inner DirectUserset message is nil (a case that can occur for in-memory models produced by TransformModuleFilesToModel). It also ensures the DSL→proto transformer always populates the DirectUserset sentinel for direct-assignment rewrites, and adds round-trip coverage for modular models.

Changes:

• Detect direct assignments by checking the Userset_This oneof case (instead of GetThis() != nil).
• Populate DirectUserset when creating direct-assignment rewrites in the DSL→proto transformer.
• Add module round-trip tests for TransformJSONProtoToDSL with and without source information.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pkg/go/transformer/jsontodsl.go	Adds isDirectAssignment and uses it throughout parsing/validation to correctly detect Userset_This even when DirectUserset is nil.
pkg/go/transformer/dsltojson.go	Ensures direct-assignment rewrites explicitly set Userset_This.This to an empty DirectUserset{} sentinel.
pkg/go/transformer/jsontodsl_test.go	Adds modular-model round-trip tests for TransformJSONProtoToDSL (with/without source info).

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