feat(plugins): generalize native platform handler registration to every gateway platform

ctx.register_platform_handler(platform, factory) — the generic surface for
plugins to wire native handlers into any platform adapter at connect()
time. Factories receive (native, adapter): the platform's client/app
object (PTB Application, discord.py Bot, slack_bolt AsyncApp, Teams App,
DingTalkStreamClient, aiohttp web.Application) or None for adapters with
no separate native object.

- BasePlatformAdapter._wire_plugin_handlers(native): shared, isolated
  invocation helper — a raising plugin cannot block a platform connect.
- All 27 connectable adapters call it: telegram/slack/teams/line/
  api_server/msgraph_webhook wire before their dispatch tables freeze;
  the rest hook at connect success.
- register_telegram_handler and get_telegram_handler_factories retained
  as thin back-compat aliases over the telegram bucket.
- Source-invariant test guarantees every adapter with connect() keeps
  calling the hook.

gateway/platforms/api_server.py

@@ -8432,6 +8432,10 @@ class APIServerAdapter(BasePlatformAdapter):
                         self.name, self._host,
                     )
 
+            # Plugin-registered native handlers (aiohttp web.Application —
+            # router routes). Wired before AppRunner.setup() freezes the router.
+            self._wire_plugin_handlers(self._app)
+
             self._runner = web.AppRunner(self._app)
             await self._runner.setup()
             # Bind directly instead of probing 127.0.0.1 first — the old