feat(messenger): scaffolding — types, format converter, cards (PR 1 of 2) (#118)

* feat(messenger): scaffolding — types, format converter, cards (vercel/chat#461)

PR 1 of 2 for the Messenger (Meta) adapter port. Non-adapter scaffolding
only; the adapter (webhook routing, Graph API, signature verification,
send/stream) lands in PR 2. Mirrors how the Telegram port was split.

Adds under src/chat_sdk/adapters/messenger/:
- types.py: MessengerAdapterConfig (app_secret / page_access_token /
  verify_token with FACEBOOK_* env fallbacks), MessengerThreadId,
  sender/recipient/attachment/message/postback/reaction/delivery/read,
  webhook entry/payload, send-API response, user profile, button +
  template-element + generic/button template payloads, MessengerRawMessage.
- format_converter.py: MessengerFormatConverter (no markdown rendering;
  stringifies AST, overrides render_postable). Mirrors upstream markdown.ts.
- cards.py: card_to_messenger (template|text), card_to_messenger_text,
  encode/decode_messenger_callback_data ("chat:{json}" prefix). Generic
  template when title/imageUrl present, Button template otherwise; tables /
  selects / radio_selects fall back to text. Caps: 20-char button titles,
  80 subtitle, 640 button-template text, 3 buttons max.
- __init__.py: exports converter, card helpers, and types only (PR 2 adds
  the adapter export).

Tests: tests/test_messenger_cards.py + tests/test_messenger_format.py
(56 tests) ported from upstream cards.test.ts / markdown.test.ts.

Design open question (#110 Q2): decode_messenger_callback_data returns the
raw string as both action_id and value for non-"chat:" payloads. Mirrored
from upstream exactly and pinned with a test + "# Divergence-candidate
(see #110)" breadcrumb; not tightened in this PR.

Refs #110, #98.

https://claude.ai/code/session_01FyMxQn2BEAzmwKS1GZczKj

* refactor(messenger): drop dead MAX_TEXT_LENGTH from cards scaffolding (review)

https://claude.ai/code/session_01FyMxQn2BEAzmwKS1GZczKj

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: patrick-chinchill

src/chat_sdk/adapters/messenger/__init__.py

@@ -0,0 +1,52 @@
+"""Messenger (Meta) adapter for chat-sdk.
+
+PR 1 of 2 (scaffolding): types, format converter, and card conversion.
+The adapter itself (webhook routing, Graph API, signature verification,
+send/stream) is added in PR 2.
+"""
+
+from chat_sdk.adapters.messenger.cards import (
+    MessengerCardResult,
+    card_to_messenger,
+    card_to_messenger_text,
+    decode_messenger_callback_data,
+    encode_messenger_callback_data,
+)
+from chat_sdk.adapters.messenger.format_converter import MessengerFormatConverter
+from chat_sdk.adapters.messenger.types import (
+    MessengerAdapterConfig,
+    MessengerButton,
+    MessengerButtonTemplatePayload,
+    MessengerGenericTemplatePayload,
+    MessengerMessagingEvent,
+    MessengerRawMessage,
+    MessengerSendApiResponse,
+    MessengerTemplateElement,
+    MessengerTemplatePayload,
+    MessengerThreadId,
+    MessengerUserProfile,
+    MessengerWebhookEntry,
+    MessengerWebhookPayload,
+)
+
+__all__ = [
+    "MessengerAdapterConfig",
+    "MessengerButton",
+    "MessengerButtonTemplatePayload",
+    "MessengerCardResult",
+    "MessengerFormatConverter",
+    "MessengerGenericTemplatePayload",
+    "MessengerMessagingEvent",
+    "MessengerRawMessage",
+    "MessengerSendApiResponse",
+    "MessengerTemplateElement",
+    "MessengerTemplatePayload",
+    "MessengerThreadId",
+    "MessengerUserProfile",
+    "MessengerWebhookEntry",
+    "MessengerWebhookPayload",
+    "card_to_messenger",
+    "card_to_messenger_text",
+    "decode_messenger_callback_data",
+    "encode_messenger_callback_data",
+]