feat(input_schema): Custom error messages

[mfori]

Solves https://github.com/apify/apify-core/issues/24045 - implements option to define custom error messages within the input schema based on the specification here: #556

Few notes to the implementation:

1. It supports only property level validation keywords like type, minLength, pattern,.. but no schema/object level keywords (required, additionalProperties)

2. It supports our custom validation keywords too (patternKey and patternValue)

3. This allows to specify any validation keyword in any property. There is no check whether the property actually supports or moreover has defined the keyword. This makes the implementation much more simpler using one errorMessage definition and reusing it in all the properties.

4. This would start working out-of-the box in the UI, but we use there custom function beautifyValidationMessage to modify the default error message for UI usecases and the custom message would just bubble through without any modification in most cases (there is regex check) - IMHO expected behavior, but mentioning it here to be sure.

[Copilot]

Pull Request Overview

This PR implements custom error messages for input schema validation, allowing developers to define field-specific error messages for validation keywords. This provides better user experience by enabling more contextual and user-friendly error messages.

Key Changes

• Added errorMessage property definition to the JSON schema with support for all standard validation keywords (type, pattern, min/max constraints, etc.)
• Implemented getCustomErrorMessage() function to retrieve custom messages from the schema based on validation paths
• Integrated custom error message lookup into both AJV-based validation (parseAjvError) and pattern validation in utilities
• Added comprehensive test coverage for all supported validation keywords and field types

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/json_schemas/schemas/input.schema.json	Added errorMessage definition and referenced it in all property type definitions (string, array, object, integer, number, boolean, resource, any, and their sub-schema variants)
packages/input_schema/src/input_schema.ts	Implemented getCustomErrorMessage() function and integrated it into parseAjvError() to prioritize custom messages over default ones
packages/input_schema/src/utilities.ts	Integrated custom error message support for pattern validation (patternKey and patternValue) in various field types
test/input_schema.test.ts	Added tests to validate schema accepts custom error messages and rejects unknown keywords
test/utilities.client.test.ts	Added comprehensive test coverage for custom error messages across all validation keywords and field types

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