fix(telegram): make buttons schema optional in message tool

The Telegram plugin injects a `buttons` property into the message tool
schema via `createMessageToolButtonsSchema()`, but without wrapping it
in `Type.Optional()`. This causes TypeBox to include `buttons` in the
JSON Schema `required` array.

In isolated sessions (e.g. cron jobs) where no `currentChannel` is set,
all plugin schemas are merged into the message tool. When the LLM calls
the message tool without a `buttons` parameter, AJV validation fails
with: `buttons: must have required property 'buttons'`.

Wrap the buttons schema in `Type.Optional()` so it is not required.

extensions/telegram/src/channel-actions.ts

@@ -1,3 +1,4 @@
+import { Type } from "@sinclair/typebox";
 import {
   createUnionActionGate,
   listTokenSourcedAccounts,
@@ -110,7 +111,7 @@ function describeTelegramMessageTool({
   if (discovery.buttonsEnabled) {
     schema.push({
       properties: {
-        buttons: createMessageToolButtonsSchema(),
+        buttons: Type.Optional(createMessageToolButtonsSchema()),
       },
     });
   }