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.

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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.

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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.