Add SMS fallback plugin (initial scaffold) and emit message:fallback hook

[jr-everstar]

Motivation

• Introduce an initial, opt-in SMS fallback mechanism to signal when outbound WhatsApp sends fail due to recipient not being registered.
• Wire a plugin extension and observability path so provider integrations (Twilio, Vonage, etc.) can be added in follow-ups.

Description

• Add a new hook event message:fallback to HookEvent and make MessageService.sendText emit message:fallback when failure matches WhatsApp-registration errors via isWhatsAppRegistrationError.
• Implement a built-in mock SmsFallbackPlugin in src/plugins/extensions/sms-fallback.plugin.ts that registers on message:failed, logs fallback intents, and exposes a small config schema (enabled, provider, apiKey, from).
• Register the sms-fallback built-in plugin in PluginLoaderService.loadBuiltInPlugins and add documentation (docs/23-sms-fallback-plugin.md) plus README entries pointing to the new docs.
• Apply small related cleanups and typing fixes across the codebase: return typing in HookManager.getRegisteredHooks, minor type/formatting adjustments in engine.factory.ts, session.service.ts (remove unnecessary casts when dispatching events), docker.service.ts PortBindings reduction, and a signature cast removal in bulk-message.service.ts.
• Update unit tests in src/modules/message/message.service.spec.ts to assert the message:fallback hook is emitted when target is not a WhatsApp account.

Testing

• Updated unit tests in src/modules/message/message.service.spec.ts to cover fallback emission and existing send flows.
• Ran unit tests with npm test and all tests completed successfully.

---

Codex Task