Generalize response status code handling

Summary

• Normalize status codes sent to nginx
  In validator.lua, added convertToValidHttpStatusCode() so the value set to ngx.status is always a valid HTTP status (100–599). Extended codes (e.g. 401013) are mapped to their 3-digit base (e.g. 401); invalid or out-of-range values fall back to 500. exitFn now uses this before setting ngx.status.

• Support extended status in error decorator
  In validatorsHandlerErrorDecorator.lua, when the response template is looked up by status, a normalized status (e.g. 401) may not match templates keyed by extended codes (e.g. 401013). The decorator now falls back to parsing the response body JSON and using error_code for the lookup, so the correct template and custom response are still applied for extended error codes.

Files changed

• src/lua/api-gateway/validation/validator.lua – status normalization and use in exitFn
• src/lua/api-gateway/validation/validatorsHandlerErrorDecorator.lua – fallback lookup by error_code from response body