feat(plugins): add standalone_sender_fn for out-of-process cron delivery

Plugin platforms (IRC, Teams, Google Chat) currently fail with
`No live adapter for platform '<name>'` when a `deliver=<plugin>` cron
job runs in a separate process from the gateway, even though the
platforms are eligible cron targets via `cron_deliver_env_var` (added
in #21306). Built-in platforms (Telegram, Discord, Slack, etc.) use
direct REST helpers in `tools/send_message_tool.py` so cron can deliver
without holding the gateway in the same process; plugin platforms
historically depended on `_gateway_runner_ref()` which returns `None`
out of process.

This change adds an optional `standalone_sender_fn` field to
`PlatformEntry` so plugins can register an ephemeral send path that
opens its own connection, sends, and closes without needing the live
adapter. The dispatch site in `_send_via_adapter` falls through to the
hook when the gateway runner is unavailable, with a descriptive error
when neither path applies. The hook is optional, so existing plugins
are unaffected.

Reference migrations land in the same change for IRC, Teams, and
Google Chat, exercising the hook across stdlib (asyncio + IRC protocol),
Bot Framework OAuth client_credentials, and Google service-account
flows respectively.

Security hardening on the new code paths:
* IRC: control-character stripping on chat_id and message body to
  block CRLF command injection; bounded nick-collision retries; JOIN
  before PRIVMSG so channels with the default `+n` mode accept the
  delivery.
* Teams: TEAMS_SERVICE_URL validated against an allowlist of known
  Bot Framework hosts (`smba.trafficmanager.net`,
  `smba.infra.gov.teams.microsoft.us`) to block SSRF; chat_id and
  tenant_id constrained to the documented Bot Framework character set;
  per-request timeouts so a slow STS endpoint cannot starve the
  activity POST.
* Google Chat: chat_id and thread_id validated against strict
  resource-name regexes; service-account refresh wrapped in
  `asyncio.wait_for` so a hung token endpoint cannot stall the
  scheduler.

Test coverage: 20 new tests covering happy path, missing-config errors,
network failure modes, and each defensive validation. Existing tests
unchanged. `bash scripts/run_tests.sh tests/tools/test_send_message_tool.py
tests/gateway/test_irc_adapter.py tests/gateway/test_teams.py
tests/gateway/test_google_chat.py` reports 341 passed, 0 regressions.

Documentation: new "Out-of-process cron delivery" section in
website/docs/developer-guide/adding-platform-adapters.md and an entry
in gateway/platforms/ADDING_A_PLATFORM.md naming the hook.

gateway/platform_registry.py

@@ -30,7 +30,7 @@ Usage (gateway side):
 
 import logging
 from dataclasses import dataclass, field
-from typing import Any, Callable, Optional
+from typing import Any, Awaitable, Callable, Optional
 
 logger = logging.getLogger(__name__)
 
@@ -125,6 +125,23 @@ class PlatformEntry:
     # resolve the default chat/room ID.  Empty = no cron home-channel support.
     cron_deliver_env_var: str = ""
 
+    # ── Standalone (out-of-process) sending ──
+    # Optional: async coroutine that delivers a message without a live
+    # gateway adapter.  Called by ``tools/send_message_tool._send_via_adapter``
+    # when ``cron`` runs in a separate process from the gateway and the
+    # in-process adapter weakref is therefore ``None``.
+    #
+    # Signature:
+    #     async (pconfig, chat_id, message, *, thread_id=None,
+    #            media_files=None, force_document=False) -> dict
+    #
+    # Returns ``{"success": True, "message_id": ...}`` on success or
+    # ``{"error": str}`` on failure.  Plugin authors typically open an
+    # ephemeral connection / acquire a fresh OAuth token, send, and close.
+    # Without this hook, plugin platforms cannot serve as cron ``deliver=``
+    # targets when the gateway is not co-resident with the cron process.
+    standalone_sender_fn: Optional[Callable[..., Awaitable[dict]]] = None
+
 
 class PlatformRegistry:
     """Central registry of platform adapters.