test(model): add unit tests for GetErrorMessage and Flag.MarshalJSON

[anxkhn]

core/pkg/model had no test coverage (0.0%) even though it is imported by 17 packages and holds the core Flag type. This adds table-driven tests for the two functions in the package that have real behaviour:

• GetErrorMessage: every known code maps to its readable message, and an unknown code falls through to Unknown error code: <code>.
• Flag.MarshalJSON: locks in the non-obvious behaviour where key is deliberately omitted from the serialized JSON, FlagSetId/Priority (json:"-") never serialize, and targeting/metadata honour omitempty.

Test-only, no production change. Brings the package to 100% coverage.

Relates to #1773.

Checklist

• Unit tests included (CONTRIBUTING requires it)
• make test-core / go test -race -short ./pkg/model/ passes (100% cov)
• make lint (golangci-lint v2.7.2) clean
• DCO sign-off present, author == signoff
• No markdown changes (markdownlint N/A)

[netlify]

❌ Deploy Preview for polite-licorice-3db33c failed. Why did it fail? →

Name	Link
🔨 Latest commit	75af3e6
🔍 Latest deploy log	https://app.netlify.com/projects/polite-licorice-3db33c/deploys/6a438909294a7800081a235e

[netlify]

✅ Deploy Preview for polite-licorice-3db33c ready!

Name	Link
🔨 Latest commit	75af3e6
🔍 Latest deploy log	https://app.netlify.com/projects/polite-licorice-3db33c/deploys/6a442f29b989acd918b3ec18
😎 Deploy Preview	https://deploy-preview-1981--polite-licorice-3db33c.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 3073c89d-dc13-4759-af5d-9248e72ab9f5

📥 Commits

Reviewing files that changed from the base of the PR and between d99a3e3 and 75af3e6.

📒 Files selected for processing (1)

• core/pkg/model/model_test.go

---

📝 Walkthrough

Walkthrough

Adds core/pkg/model/model_test.go with unit tests covering GetErrorMessage error-code-to-string mapping and Flag JSON marshaling, verifying field inclusion/exclusion behavior.

Changes

Model Unit Tests

Layer / File(s)	Summary
GetErrorMessage and Flag JSON marshaling tests core/pkg/model/model_test.go	Tests verify GetErrorMessage returns expected strings for known, unknown, and empty error codes; Flag JSON output omits key, FlagSetId, and Priority while including state, defaultVariant, source, variants, targeting, and metadata; a separate test confirms optional fields are omitted when empty.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 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
Title check	✅ Passed	The title accurately summarizes the main change: adding unit tests for GetErrorMessage and Flag.MarshalJSON.
Description check	✅ Passed	The description is directly related to the change and matches the added test coverage for core/pkg/model.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (2.12.2)

level=error msg="[linters_context] typechecking error: pattern ./...: directory prefix . does not contain main module or its selected dependencies"

---

_{Comment @coderabbitai help to get the list of available commands.}