Use Forge as the default Serverless Training endpoint

[Kovbo]

Use https://forge.coreweave.com/api/training/v1 as the default Serverless Training base URL now that the Forge route is deployed. Explicit base_url overrides continue to work.

Validation: existing focused Serverless tests, Ruff, and targeted type checks passed during the endpoint update. Live Forge health/model discovery and RL/SFT request-validation checks passed. No training job or long-running inference request was run; the full unit suite could not collect locally because optional training dependencies such as Torch are absent.