fix(channel): detect @all in rich-text post messages (fixes #138)

[Xuxchloris]

Fixes #138

Problem

For post (rich-text) messages that mention everyone, InboundMessage.mentioned_all is always False, so PolicyGate never emits policy_mention_all_blocked and a bot configured with respond_to_mention_all=False still responds to broadcast @all posts. Plain-text @all messages are handled correctly.

Two compounding issues in the post inbound path:

1. Feishu does not populate mentions[] for post @all — the signal lives in the post AST as an at node with user_id == "all". _flatten_post_text rendered that node to the literal @<user_name> (@Everyone), which neither text_has_mention_all (looks for the @_all placeholder) nor parse_at_tags (looks for <at> tags) can match.
2. _flatten_post_text only handled locale-keyed post payloads ({"zh_cn": ...}); the classic single-locale shape {"content": ...} (the one used in the issue reproduction) returned an empty text, so the probe never even ran.

Changes

• lark_oapi/channel/normalize/registry.py
  • _flatten_post_text now accepts both wire shapes: locale-keyed posts and the direct {"content": ...} shape.
  • An at node whose user_id == "all" (top-level or inside id) renders as the @_all placeholder, so the pipeline's text_has_mention_all probe fires and resolve_mentions rewrites it to the human-visible @all.
• lark_oapi/channel/tests/test_post_mention_all.py (new): pipeline-level tests for both post shapes (@all → mentioned_all=True, rendered @all; a regular @<user> mention → mentioned_all=False, name rendered).

Verification

• python -m pytest lark_oapi/channel/tests — 662 passed; the single failure (test_upload_error_propagation.py::test_gather_buffer_missing_local_file_raises_upload_failed) is a pre-existing Windows-only path-escaping assertion unrelated to this change (CI runs on Linux).

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