Revert base_url to a single string (drop multi-client)

[mikasenghaas]

Roll back base_url to a plain str on the orchestrator client config. The multi-client/list-of-URLs path isn't needed and adds config + plumbing complexity for no current benefit.

Scope

• Change base_url back to str (single endpoint).
• Remove multi-client fan-out / selection logic and any related config knobs.
• Update configs, tests, and skills/docs that reference the list form.

[mikasenghaas]

Audited deployments and this is not easy to support right now — multi-node RL via src/prime_rl/templates/multi_node_rl.sbatch.j2 actively relies on multi-URL CLI args:

• --client.base-url $INFER_URLS — one router URL per inference replica; multi-URL when num_infer_replicas > 1 (added in feat: disaggregated prefill-decode inference with multi-replica support #2030 for disaggregated PD + multi-replica).
• --client.admin-base-url $ADMIN_URLS — one URL per backend pod; multi-URL essentially always in multi-node RL.

So multi-replica / disaggregated-PD jobs depend on base_url: list[str] and admin_base_url: list[str] | None today. Reverting to single strings would silently break those jobs at config-parse time. Closing the PR (#2560); leaving this issue open as a follow-up if/when the slurm template is restructured to front the per-replica routers behind a single endpoint.