feat(FN-3163): test suggestion route error contract in roadmap routes

Adds an assertion for the suggestion route error contract in the roadmap routes test file, completing the test coverage for that endpoint.

Fusion-Task-Id: FN-3163

docs/PLUGIN_AUTHORING.md

@@ -456,7 +456,8 @@ const plugin: FusionPlugin = {
 
 ### Route Mounting
 
-Routes are mounted at `/api/plugins/{pluginId}/{path}`:
+Routes are mounted at `/api/plugins/{pluginId}/{path}`.
+Route handlers receive the same loader-built `PluginContext` used by hooks/tools, including real `taskStore`, plugin `settings`, `logger`, `emitEvent`, and engine-injected `createAiSession` (when available):
 
 - Plugin ID: `fusion-plugin-notification`
 - Route path: `/status`