test: support hermetic local API reference

[apcha-oai]

Allow the SDK mock server to use a locally available API reference when present, while preserving the existing network-based fallback.

[Copilot]

Pull request overview

Updates the scripts/mock helper to support hermetic/local usage by preferring a locally checked-in/generated OpenAPI spec when available, while preserving the existing .stats.yml URL-based fallback.

Changes:

• If no explicit spec path/URL arg is provided, prefer api_reference/openapi.transformed.yml when present.
• Verify the local spec matches generation metadata by comparing its MD5 hash to openapi_transformed_spec_hash from .castiron.stats.yml.
• Retain existing behavior of reading openapi_spec_url from .stats.yml when local inputs aren’t available.

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