Move 'validate_params' into Task trait

[Aaron1011]

This allows the implementation to be overridne by consumers of durable

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Note

Low Risk
Small API surface change with a default implementation that preserves current behavior; risk is limited to downstream compile updates and any custom overrides affecting validation outcomes.

Overview
Moves task parameter validation into the public Task trait by adding a default validate_params method that deserializes JSON into Self::Params.

Updates TaskWrapper/ErasedTask to delegate validation to the underlying task, enabling per-task custom validation before spawn_by_name accepts and enqueues work.

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