Lang-SDK runtimes: one transport for native Dag parsing and the task-handler probe #74058

Native Dag parsing (#74035, LangSDKDagFileProcessorProcess in dag_processing/lang_sdk_processor.py) and the task-handler probe (#73974, LangSDKTaskHandlerProcessorProcess in dag_processing/task_handler_processor.py) each carry their own copy of the runtime transport: the listeners, connection checks, frame reading, request handling, timeouts and the runtime kill, plus their own StartLangSDKRuntime and LangSDKRuntimeSchemaVersion models. Once both are on main:

• Extract one shared base and move both processes onto it, with a single tests/unit/dag_processing/fake_lang_sdk.py.
• Keep that commit free of behaviour changes. Then, as separate changes, give native parsing what the probe already has: a reported start failure (LangSDKRuntimeStartFailed), non-blocking frame reads, killing what a runtime leaves behind, and cleanup when a start fails after the fork. The parent-death signal stays opt-in, because a native parse may be started from a short-lived thread.
• Results from a Lang-SDK runtime must not carry task_handler_bindings or probed_artifacts; the manager ignores those fields from a runtime.

Drafted with Claude Code (Opus 5.5).