runtime: enforce RawJSON tool boundary contracts (#91)

* runtime: enforce RawJSON contracts across tool boundaries

Switch workflow-boundary tool payload/result fields from json.RawMessage to rawjson.RawJSON across runtime paths and provider adapters so empty payloads no longer break Temporal encoding. Also stabilize codegen/golden header handling and add rawjson boundary tests to lock marshal/unmarshal semantics.

* tests/runtime: satisfy lint rules for RawJSON follow-up

Replace unnecessary RawJSON conversion in planner stream consumption and align assertions with testifylint expectations in runtime/rawjson tests so CI lint passes on the follow-up branch.

* runtime: preserve canonical child ToolArgs contract

Copy canonical tool payload bytes directly into child run ToolArgs instead of re-encoding via payload codec, which could silently drop args and produce null ToolArgs.

Author: raphael