Add 'cached' field to durable.task.step span

[Aaron1011]

Note

Low Risk
Low risk: telemetry-only change that adds a span field and records whether step() returned a cached checkpoint result. No changes to step execution, checkpointing, or data persistence logic.

Overview
Adds a new cached field to the durable.task.step tracing::instrument span and records it at runtime as true when a checkpointed step returns a cached value and false otherwise.

This is gated behind the telemetry feature and does not alter step behavior beyond additional span metadata.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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