feat(ads-client): add contract tests to validate types against MARS

[Almaju]

Add two layers of contract validation:

1. Unit contract tests (run on every cargo test, zero cost):

   • ad_response.rs: spec-accurate JSON fixtures for Image, Spoc, and Tile deserialization, including tests that document intentional deviations (click/impression required by us but optional in spec; attributions field not modelled but silently ignored)
   • ad_request.rs: serialization shape tests and taxonomy string validation

2. Integration contract tests against staging (ignored by default):

   • Three new #[ignore] tests in integration_test.rs that hit ads.allizom.org and assert all required spec fields are present and non-empty for each ad type (Image, Spoc, Tile)
   • Run with: cargo test -p ads-client --test integration_test -- --ignored

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