feat: add support for decoding strategies in health metadata

[shackmann]

This is a basic but breaking change. The Python SDK works only with a model instance that exposes the required field.

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Note

High Risk
Breaking health wire contract: any deployment without a valid decoding_strategy fails compatibility checks at health time, blocking all SDK usage until the service is upgraded.

Overview
Breaking V1 health contract: /healthz must now include decoding_strategy. The SDK parses it on HealthMetadata, exports SUPPORTED_DECODING_STRATEGIES (parallel_dense, parallel_scalable, autoregressive), and validate_service_metadata() rejects missing, empty, or unknown values with UnsupportedServiceContractError—the same path used by JointFMClient.health() before forecasts run.

Docs, the service-health notebook, fixtures, and tests were updated to match. Deployments that omit the field or run an older inference image will fail health checks until they advertise a supported strategy.

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