test: finish the Remote-result and picker-split test migrations

Every generated Remote method resolves to `RemoteResult<T>`, so the Gateway
client spec asserts the ok and error branches instead of the unwrapped value
and a throw, and the generator fixtures declare the wrapper in the consumer
face they typecheck. The RPC-failure test splits into the Host error carried
verbatim in the error branch plus a transport throw folded into it.

The runtime client, ui-command and ui-plan benches answer the generated
commands Remote through its result branches and provide the `remote.commands`
namespace their plugins now inject; the ui-command bench also serves the `$on`
the service subscribes on construction.

The directory-picker chooser mounts a backend and its surface as a pair, so the
real-Loader composition serves both surface packages and asserts each entry
arrives and leaves with its backend.

packages/client/runtime/tests/client-apply.spec.ts

@@ -14,7 +14,7 @@ import type { ConversationNodeDefinition } from '../src/client/contract/conversa
 import { Session } from '../src/client/sessions/session.ts'
 import type { SessionsService } from '../src/client/sessions/service.ts'
 import type { WorkspacesService } from '../src/client/workspaces/service.ts'
-import { FakeApiClient, ok } from './fake-api.ts'
+import { FakeApiClient, fakeRemote, ok } from './fake-api.ts'
 
 interface Bench {
   ctx: Context
@@ -45,6 +45,7 @@ async function mount(): Promise<Bench> {
   }
   ctx.reflect.provide('connection', handle)
   ctx.reflect.provide('remote', {})
+  ctx.reflect.provide('remote.commands', fakeRemote().commands)
   await ctx.plugin(RuntimeClient).await()
   return bench
 }