Add comprehensive validator error message test suite (#266)

* Add ValidatorMessageFormatSpec to validate error message backward compatibility

This spec serves as a baseline for validator 1.0.76 error message format.
It tests all major validator types to ensure error messages remain consistent
after upgrading json-schema-validator to 1.5.8.

All 20 tests pass with validator 1.0.76, confirming that the test expectations
accurately capture the current error message behavior.

* Add comprehensive validator error message backward compatibility test suite

Added ValidatorMessageFormatSpec with complete coverage of all JSON Schema
Draft 4 validation keywords supported by json-schema-validator 1.0.76.

## Coverage (26 validators)
- Numeric: minimum, maximum, multipleOf, exclusiveMinimum, exclusiveMaximum
- String: minLength, maxLength, pattern
- Array: minItems, maxItems, uniqueItems, items, additionalItems
- Object: minProperties, maxProperties, required, additionalProperties, patternProperties, dependencies
- Type: type, enum
- Composition: allOf, anyOf, oneOf, not
- Format: format

## Implementation Details
- All test methods numbered sequentially (e1-e26) matching spec definition order
- All assertions use beEqualTo with exact error messages for precise matching
- No fuzzy contain() assertions - maximum confidence in error message stability

This baseline test suite will be used to verify backward compatibility
when upgrading from json-schema-validator 1.0.76 to 1.5.8.

Author: oguzhanunlu