Add linting rules for AEP-126: Enumerations

[shakrav2]

Summary

Implements linting rules for AEP-126 (Enumerations) to validate enum field usage in OpenAPI specifications.

Closes #58

Rules Added

This PR adds 6 new linting rules:

1. aep-126-enum-type-string (error) - Enums must use type: string, not integer or number
2. aep-126-enum-case-consistent (warn) - All enum values must use consistent case formatting
3. aep-126-enum-null-first (warn) - Nullable enums must have null as the first value
4. aep-126-enum-nullable-declaration (error) - Enums containing null must declare nullable: true
5. aep-126-no-standard-value-enums (warn) - Warns against enumerating standard codes (ISO 639, ISO 3166, ISO 4217, IANA)
6. aep-126-enum-has-description (info) - Enum fields should include a description

Implementation Details

• Custom Functions: 4 new Spectral functions in functions/aep-126-*.js
• Tests: 38 test cases across 6 test files in test/0126/
• Documentation: Complete rule documentation in docs/0126.md
• JSONPath Pattern: Uses parent selector $..[?(@property === 'enum')]^ to safely handle enums containing null values

Test Results

✅ 38/38 tests passing (100%)
✅ Code coverage: 80-85% for all custom functions
✅ Linter: All checks passing

Testing

```bash
npm test -- --testPathPattern=0126
```

Checklist

• All 6 rules implemented in `aep/0126.yaml`
• Custom functions created with proper error handling
• Comprehensive tests written (38 tests)
• Documentation added in `docs/0126.md`
• `spectral.yaml` and `docs/rules.md` updated
• All tests passing
• Code coverage >80%
• Linter passing