Add builder register helper methods that take &mut self

[Aaron1011]

This will make it easier to register things in tensorzero/tensorzero

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Note

Low Risk
Low risk API addition confined to builder ergonomics; behavior is unchanged aside from routing existing register_instance through the new shared implementation.

Overview
Adds DurableBuilder::register_mut and DurableBuilder::register_instance_mut so tasks can be registered using &mut self instead of the consuming builder pattern.

Refactors register_instance to delegate to the new mutable helper, keeping the same duplicate-name validation and registry insertion logic while returning Self for chaining.

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