Register tasks in DurableBuilder rather than Durable client

[Aaron1011]

This enforces that no new tasks can be registered once a Durable client has been constructed. As a result, we can avoid using an RwLock around the TaskRegistry

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Note

Medium Risk
This is an API-breaking change that moves task registration to client construction and alters core worker spawn/dispatch paths by removing the registry RwLock. Risk is moderate because it touches the primary client/worker lifecycle, though behavior is mostly a structural refactor.

Overview
Task registration is moved from Durable to DurableBuilder. The async Durable::register* APIs are removed; tasks are now added via DurableBuilder::register/register_instance before build/build_with_state, and spawn_by_name_with validates against this immutable registry.

Runtime registry access is simplified and lock-free. Durable now holds an Arc<TaskRegistry> (no RwLock), and worker handler lookup uses direct registry reads. All benches and tests are updated to construct clients/builders with pre-registered tasks (including new BenchContext::with_builder / new_builder patterns).

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