Refactor process_running_jobs background task (#3648)

* Extract running job helper functions

* Split running job state handlers

* Introduce running job context

* Extract running job startup context

* Use keyword args for running job RPCs

* Defer running job payload loading

* Fix missing jrd update

* Extract backend provisioning helpers

Author: r4victor

AGENTS.md

@@ -18,6 +18,8 @@
 - Python targets 3.9+ with 4-space indentation and max line length of 99 (see `ruff.toml`; `E501` is ignored but keep lines readable).
 - Imports are sorted via Ruff’s isort settings (`dstack` treated as first-party).
 - Keep primary/public functions before local helper functions in a module section.
+- Roughly keep function definitions in the order they are referenced within a file so call flow stays easy to follow.
+- Prefer early returns over nested `if`/`else` blocks when they make the control flow simpler.
 - Keep private classes, exceptions, and similar implementation-specific types close to the private functions that use them unless they are shared more broadly in the module.
 - Prefer pydantic-style models in `core/models`.
 - Document attributes when the note adds behavior, compatibility, or semantic context that is not obvious from the name and type. Use attribute docstrings without leading newline.