feat: add OpenAPI mapping validation and schema support

[ndenny]

This pull request introduces schema-based validation for the OpenAPI mapping file, ensuring that mapping documents conform to a defined structure before processing. It also updates documentation and tests to reflect and support this new validation step, and adds a JSON schema file for the mapping format. The main changes are grouped below:

Schema validation and enforcement:

• Added src/openapi-mapping.schema.json defining the allowed structure for OpenAPI mapping files, including properties like server_url, default_tags, include, exclude, defaults, operations, and workflow.
• Implemented runtime validation of the OpenAPI mapping file in src/index.js using ajv and the new schema, with fast failure and detailed error reporting. The mapping file is now always validated before transformation. [1] [2]
• Added additional runtime checks in buildProjectFromOpenApi (src/openapi.js) for key mapping properties (include, exclude, operations, defaults) to ensure correct types are provided.

Documentation updates:

• Updated README.md to document schema validation, including schema references, YAML language server integration, and a new section describing validation behavior and error output. [1] [2] [3]

Test coverage:

• Added new tests in test/openapi.test.js to verify that improper types for include, operations, and defaults in the mapping file result in appropriate errors.

Example and schema usage:

• Updated example mapping files (api/apim-mapping.yaml) and documentation examples to reference the schema for editor integration and validation. [1] [2]

These changes collectively ensure that mapping files are validated early, errors are clear, and documentation and examples are up-to-date with the new validation process.