Support Video in OpenRTB requests #361

Context

The auction request model has MediaType::Video, but the current Prebid OpenRTB request builder filters formats to MediaType::Banner and drops an impression with no valid banner formats (crates/trusted-server-core/src/integrations/prebid.rs). The shared config-first OpenRTB path also needs review before choosing an implementation. The original issue has no description, so it does not establish the intended video inventory or provider contract.

Desired outcome

Support a defined video impression in an OpenRTB request without silently sending it as banner inventory or dropping it. Keep existing banner behavior intact.

Scoped work

• Decide and document the first supported video use case: request source, OpenRTB version and imp.video fields, supported providers, and how a video response will be consumed. Identify any requirements for player context, privacy signals, and mixed banner/video slots. Keep response rendering outside this issue unless the chosen use case requires it.
• Trace the browser or API auction input through AdSlot and the active provider builders. Add the needed typed video input and serialization for the selected route.
• Define validation for missing or invalid video parameters and explicit behavior for providers that do not support video.
• Add request-shape and regression tests for video-only and mixed inventory, including unsupported or malformed input.

Done when

• The chosen video contract and provider scope are documented with a representative fictional request shape.
• A valid in-scope video slot produces the agreed OpenRTB video impression, with its required fields preserved to the provider request.
• Invalid or unsupported video input has a deliberate, tested outcome; banner requests remain unchanged.
• Any response-handling or rendering work needed for the chosen use case is either covered and tested here or tracked in a linked follow-up.

Open decisions

The title does not specify which video fields, provider, player integration, or response format should be supported first. Set those decisions before implementation.