Create test run

Create and start a new test run against a test set. Optionally apply a prompt-template tweak by setting config.prompt_template on the request body. Fetch the inferred template first via GET /v1/test-sets/{test_set_id}/prompt-template, edit the literal text around the {{var_N}} placeholders in template.messages[i].content, then submit the edited template here. The runtime extracts per-request values from each captured request using the slot regexes and substitutes them back into the edited content before sending each request to the model. Leave config.prompt_template unset (the default) to run the test set against the captured requests as-is.