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/ui-command/tests/browser-plugin.spec.ts

@@ -33,7 +33,9 @@ async function bench() {
     scopeOf: (c: Context) => scopeOf(c),
   })
   const commandsRemote = { list: () => Promise.resolve([]) }
-  ctx.provide('remote', { commands: commandsRemote })
+  // The service subscribes its cache-invalidation events on construction, so
+  // the Remote face needs `$on` even where this spec dispatches none.
+  ctx.provide('remote', { commands: commandsRemote, $on: () => () => {} })
   ctx.provide('remote.commands', commandsRemote)
   await ctx.plugin(SlotsService).await()
   ctx.slots.register({