feat(linter): add forbid_empty_enum rule

[Vaibhav701161]

Summary

This PR introduces a new lint rule: forbid_empty_enum.

The rule detects schemas declaring an empty enum array.

{
  "enum": []
}

An empty enum makes the schema unsatisfiable because no instance can ever match it.
This is usually an authoring mistake and should be reported to the user.

The rule emits a diagnostic when:

• enum exists
• the value is an array
• the array is empty

The diagnostic points to the enum keyword location.

This rule is non auto-fixable because the correct resolution depends on the schema author's intent.

---

Implementation

The rule follows the existing alterschema linter architecture:

• implemented as a header-only rule in
  src/extension/alterschema/linter/forbid_empty_enum.h
• registered in alterschema.cc
• added to the SOURCES list in the alterschema CMake configuration

The rule checks:

• schema node is an object
• enum keyword is defined
• enum is an array
• array length is zero

If these conditions are met the rule returns:

APPLIES_TO_KEYWORDS("enum")

---

Tests

Tests were added across supported dialects using the existing lint testing utilities.

The following scenarios are covered:

1. Rule fires for an empty enum
2. Rule does not fire when enum contains values
3. Rule does not fire when enum is absent
4. Rule fires inside nested subschemas (e.g. inside properties)

All tests follow the patterns used by existing lint rule tests.

---

Related Work

This rule addresses one of the pending lint rules listed in:

Refs: #1975

[cubic-dev-ai]

No issues found across 8 files

[Vaibhav701161]

@jviotti Could you please review this pull request when you have a moment?

If the approach looks good, I’d be happy to continue implementing additional linting rules listed in #1975.

[jviotti]

Ah, cool. Thanks for raising this up! I'm now thinking whether this can be auto-fixable or not. Here is the thing:

• The meta-schema technically allows empty enum: https://json-schema.org/draft/2020-12/meta/validation
• But none of the tests exercises it: https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/main/tests/draft2020-12/enum.json

Can you send a PR to the tests checking what happens with an empty enum and cc me? I guess it means validate nothing, but sounds like something to discuss with the group. Depending on that outcome, maybe we can make it auto-fixable? For example:

• If it does nothing, we can just auto-fix by removing the enum
• If it validates nothing, you can turn the entire current subschema to false

[Vaibhav701161]

Good point !
I'll open a PR in the JSON Schema Test Suite adding test cases for an empty enum so we can clarify the intended behaviour, and cc you there.

If the tests confirm that an empty enum makes the schema unsatisfiable, we could potentially make this rule auto-fixable by replacing the subschema with false. If the intended behaviour is that it has no effect, then the auto-fix could simply remove the enum keyword.

I'll report back once the test suite PR is up.