test(ads-client): add OpenAPI schema contract tests against MARS spec

[Almaju]

Summary

• Adds openapi.json snapshot of the MARS API spec
• Adds unit contract tests in ad_request.rs and ad_response.rs that validate our serialized types against the schema (runs on every cargo test, zero network cost)
• Explicitly documents intentional deviations: click/impression callbacks are optional in spec but required by us; attributions field is not modelled and silently ignored

Split from #7263

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