Refactor TaskContext to hold and use a Durable client

[Aaron1011]

We now delegate to the existing 'spawn_by_name' (wrapping it in a checkpoint in TaskContext). This lets us re-use all of the existing logic, including the OpenTelemetry context propagation logic. This will give us a tree structure in OpenTelemetry - subtasks will use their parent task as the parent trace

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Note

Modernizes task execution plumbing to flow a Durable client instead of raw pool/queue/registry, simplifying APIs and reusing client logic.

• TaskContext now stores Durable and uses it for DB ops, telemetry fields, and spawn_by_name (checkpoint-wrapped), injecting parent_task_id for cascade cancellation/trace trees
• Worker accepts/clones Durable, uses it for claiming/executing tasks, registry lookups, metrics, and pool access; start path simplified
• Durable adds clone_inner, exposes registry(), switches pool ownership to AtomicBool, and serializes parent_task_id via spawn options
• New TaskError::SubtaskSpawnFailed with JSON serialization; SpawnOptions gains crate-private parent_task_id
• Tests updated to new spawn/options patterns (mutable defaults) and refactored APIs

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