Summary

Fixes two distinct classes of Feishu message delivery failures where markdown content causes messages to be swallowed or appear blank on the client.

Problem 1: Fenced code block boundary mis-detection

_build_markdown_post_rows used naive regex ^```\s*$ to detect open/close fences. This fails for:

• Code blocks with 4+ backticks (e.g. ``````) — inner ``` lines were mistaken for close fences
• Tilde fences (~~~) — not recognized at all
• Language tags with backticks (e.g. ``` `) — close regex didn't match

Result: Code blocks truncated early or never closed, corrupting post JSON → "消息类型不合法" or blank messages.

Problem 2: Markdown tables sent as post appear blank

When a message contains both **bold** (matching _MARKDOWN_HINT_RE) and a markdown table, the entire content is sent as post type. Feishu's md tag does not render markdown table syntax — the message appears completely blank on the client.

Result: Users cannot see anything for table-containing messages.

Changes

1. Fence boundary detection — replaced naive regex with fence_char + fence_len state tracking (CommonMark compliant). Open/close fences must use the same character and closing length must be ≥ opening length.

2. Post→text fallback — ty-agent lacked fallback logic entirely; hermes only caught English errors. Expanded _POST_CONTENT_INVALID_RE to cover Chinese error messages and added robust fallback to plain text.

3. Table detection — added _MARKDOWN_TABLE_RE to detect markdown table syntax (|...| + separator line). Forces text type for table content, ensuring it displays as plain text instead of being swallowed.

Test Plan

• Unit tests for fence boundary (6 cases including 4-backtick nesting, tilde fences)
• Verified _MARKDOWN_TABLE_RE correctly identifies tables while avoiding false positives on non-table markdown
• Confirmed text payload format for table content is valid JSON
• Live test: table message successfully delivered and visible on Feishu client
• Restarted gateway and verified running code has fence_len + _MARKDOWN_TABLE_RE

Backwards Compatibility

No breaking changes. Messages without tables or code blocks behave identically. Table messages now consistently use text type instead of unreliable post type.

Related Issues

Fixes message-swallowing bugs in Feishu adapter when sending markdown tables or nested fenced code blocks.