fix(gateway/feishu): always route outbound via post + md tag

[0x5446]

Summary

FeishuAdapter._build_outbound_payload previously used two regex heuristics to choose between msg_type=text and msg_type=post. Both heuristics produced visible misrendering:

1. _MARKDOWN_TABLE_RE force-downgraded any message containing a GFM table to msg_type=text. The downgrade made the entire message — headers, bold, lists, code, the table itself — render as raw markdown source on both Lark (international) and Feishu (CN) clients.
2. _MARKDOWN_HINT_RE over-matched on incidental punctuation (_ * # > in shell output, stack traces, prose) and under-matched on legitimate markdown that didn't fit its narrow patterns — yielding inconsistent styling between similar-looking messages.

Fix

Always route outbound messages through post + the md tag.

Empirical testing on current Lark and Feishu clients shows the md tag is a strict superset of msg_type=text:

Content type	md tag result
Plain prose	renders identically to text
URLs	auto-linked
Mentions	work
Emoji	display correctly
Special chars (_*#>)	literal
Multi-line	preserved
Markdown tables	render as native tables ✅

The outer _feishu_send_with_retry still catches the rare "content format of the post type is incorrect" API error and falls back to msg_type=text, so the safety net stays.

Drops both regex constants and the conditional branching — one code path, one render mode.

Tests

• tests/gateway/test_feishu.py: 200/200 pass
• Added regression test test_outbound_payload_routes_table_to_post_not_text — pins the table-bug fix
• Added regression test test_outbound_payload_plain_text_uses_post_md — pins the strict-superset claim
• Updated existing text-mode assertion that no longer applies

Verification

Validated end-to-end through the live gateway adapter (not a raw API bypass) on Lark international — plain text, special chars, URLs, multi-line+emoji, and a markdown table all rendered correctly.

[alt-glitch]

Competing fix for the long-standing Feishu table rendering issue family (#9549, #27469). Also competing with open PRs: #26108, #27739, #27922, #22259, #16474. This PR takes the most aggressive approach — always routing through post + md tag and removing the regex heuristics entirely.

[teknium1]

Thanks for the focused Feishu rendering investigation. The current table-to-text behavior is still present, but this patch needs rework before it can be safely salvaged.

Problems

• The patch changes the removed gateway/platforms/feishu.py; current main moved the adapter to plugins/platforms/feishu/adapter.py in 560010547. Current tests likewise import plugins.platforms.feishu.adapter (for example tests/gateway/test_feishu.py:489).
• The proposed universal post route is not protected against accepted-but-misrendered payloads: the fallback in plugins/platforms/feishu/adapter.py:1913-1936 only handles explicit post-format rejection. Linked discussion on [Feishu] Markdown tables not rendering in Feishu messages #9549 reports post md silently dropping GFM tables, while this PR's tests validate payload serialization rather than Feishu/Lark rendering.

Suggested changes

• Port the changes and tests to the bundled-plugin paths.
• Reproduce the claimed post-md behavior against current Feishu/Lark clients, or coordinate the table route with the Schema 2.0-card approach under feat(feishu): Schema 2.0 markdown card for GFM tables, code blocks, and inline formatting #32488.

This is an automated hermes-sweeper review.