Unify runtime action dispatch boundary

[shuxueshuxue]

Summary

• unify runtime notification and runtime thread-input dispatch through runtime_action.py
• remove trivial one-action planner wrappers and split queued thread-input actions out of runtime-gateway actions
• add overload-backed type contracts for action dispatch results and envelope planning

Verification

• uv run pytest tests/Unit/backend/web/services/test_runtime_action_dispatcher.py tests/Unit/backend/web/services/test_runtime_notification_action.py tests/Unit/integration_contracts/test_threads_router.py -q
• uv run ruff check backend/threads/chat_adapters/runtime_action.py tests/Unit/backend/web/services/test_runtime_action_dispatcher.py
• uv run pyright backend/threads/chat_adapters/runtime_action.py tests/Unit/backend/web/services/test_runtime_action_dispatcher.py
• uv run pytest tests/Unit/backend tests/Unit/integration_contracts -q

Notes

• five coherent commits are preserved for review
• no DSL, scheduler, retry queue, schema change, local daemon, polling, or transport change