worker: add a plan setup wrapper

[geoffreyclaude]

Problem\n\nA host can attach query ownership while building a worker session, but the material set-plan work happens later: physical-plan decoding, worker plan hooks, and the initial sampler kickoff currently have no common wrapper. That leaves integrations such as df-executor unable to activate their query CPU owner while this work runs.\n\n## Implementation\n\n- Add a default, object-safe WorkerSessionBuilder::run_plan_setup hook.\n- Run physical decode, all worker plan hooks, and first-sampler kickoff inside that hook.\n- Preserve the hook through MappedWorkerSessionBuilder.\n- Stop the wrapper before task publication and later execution, including dynamic producer-head work.\n- Document the extension point and its successful-callback contract.\n\nThe default implementation is behavior-preserving. No wire format, plan shape, scheduling boundary, or execution ordering changes.\n\n## Validation\n\nFocused unit tests prove default execution, callback-error propagation, and mapped-builder delegation. Worker integration tests prove a custom codec decode, both plan hooks, and real sampler kickoff observe the wrapper, and that wrapper errors reach the query. The integration-feature all-target check, clippy with warnings denied, rustdoc tests, formatting, and whitespace checks pass.\n\n## Documentation\n\nThe worker guide now explains which setup phases the hook covers and that mapped builders retain the wrapper.

[geoffreyclaude]

Closing at the host integration owner's request. Gate B will be implemented entirely in dd-source without changing datafusion-distributed.