feat: align SDK client and runtime contracts

[kinzhi]

What changed

• enforce run idempotency headers and bounded response decoding
• align run-event, child-run, and registration DTOs with Core
• add creator registration and durable ensure_agent
• converge terminal assignment errors and bound cancellation acknowledgement
• add Core client and registration contract copies and tests

Why

Close wire-contract and Runtime behavior gaps against the Go and JavaScript SDKs while preserving the existing public flow.

Validation

• pytest (91 passed, 1 skipped)
• ruff check .
• sdist and wheel build
• git diff --check