feat(scenario): add context.assert() API for script assertions

- Add context.assert(fn, description) for function assertions
- Logs outcome on success/failure, throws on assertion failure
- Bump package version from 1.8.0 to 1.9.0

docs/scenario.md

@@ -35,6 +35,7 @@ Available in scope:
 - `context.getStepOutput(order)`: prior step output by absolute order (`1`, `2`, ...) or relative (`-1` = previous step)
 - `context.dumpDom(selector?)`: simplified DOM snapshot
 - `context.log(...args)`, `context.warn(...args)`, `context.error(...args)`: structured step logs
+- `context.assert(() => boolean, description)`: runs the arrow function and requires it to return `true`. Logs `description` (as a `log` entry on success, as an `error` entry on failure or exception), then throws to stop the step if the function returned anything other than `true` or threw
 - `context.runSnippet(name, ...args)`: execute stored snippet code with the same context
 - `context.getScenarioFiles(opts?)`: list files uploaded to the scenario (`limit`, `offset`)
 - `context.downloadFile(url, opts?)`: fetch `url` and save the result as a run artifact (requires a real scenario run — throws when invoked ad hoc, e.g. via the `exec_code` MCP tool). `opts` may include `method`, `headers`, `body`, `filename`. `url` also accepts a `data:` URI (`data:<mediaType>;base64,<data>` or `data:<mediaType>,<percent-encoded data>`) to save content generated in-script (e.g. a credential JWT) directly, without an actual network fetch.