feat: expose BDD test cases in the v2 API

Adds feature-file import and validation for cases, the per-case feature
document, run example-result listing, and example/step execution
updates. Case detail attaches BDD content, case patches accept Gherkin,
and run detail carries snapshots and example results. Contract metadata
defines the path parameters and Test management tags for the new raw
routes, and the committed OpenAPI spec is regenerated from it.

Author: stefan-ernst