Provider model discovery

Providers may implement the optional ProviderModelDiscovery contract to publish bounded model metadata through GET /v1/providers/{provider}/models. The endpoint performs metadata discovery only; it never executes inference.

The runtime applies its own one-second default discovery deadline, configurable with TRUSSIUM_RUNTIME__MODEL_DISCOVERY_TIMEOUT_SECONDS. Unsupported providers, timeouts, malformed results, and provider failures return safe unavailable reasons without raw exception text.

Model identifiers and ownership values are bounded and validated. Credentials, endpoints, prompts, responses, provider payloads, and implementation details are excluded from the response.