* Use `call_api_method()` for expected-to-fail API calls only
* Drop `get_value()` altogether (with type stubs it brings more harm
  than good)

* Forbid running multinode tasks on non-cluster fleets

* Guarantee fleet cluster placement with concurrent provisioning

* Fix tests

* Fix fleet lock on sqlite

* Simplify fleet selection code

* Recommend AsyncExitStack

* Use is_db_sqlite() and is_db_postgres()

- Allow the proxy service clients to read their timeout from DSTACK_SERVICE_CLIENT_TIMEOUT, falling back to the previous 60s default so operators can tune it without code changes.
  - Document the new environment variable in the server reference so the knob is discoverable.

  AI-Assistance: This PR description was prepared with help from codex.

…nly) (#3276)

* Support placement groups for elastic fleets

* Test placement group creation

* Update run_job() signatures

* Fix non-master jobs stuck with waiting master job

* Simplify tests

* Fix run_jobs signature

Part-of: #3126

Extends Kubernetes backend gateway support to providers that provide
IPs, not domain names, for externally-accessible Services.

Tested on Nebius (mk8s) and Google Cloud (GKE).

Part-of: #3126

Revert detecting and passing `default_repo_branch` for backward comnpatibility of the `datack-runner`