fix(feishu): wrap markdown tables in code_block tags for post rendering

[asdlem]

Problem

Feishu's post md tag drops markdown tables silently, while the old workaround (forcing text mode) loses bold, italic, code, links, and lists.

Solution

1. _convert_tables_to_code_blocks — wraps markdown tables in ``` fences
2. _build_markdown_post_rows — detects ``` fences and emits Feishu's native code_block tags (instead of `md`)
3. _choose_outbound_msg_type — module-level routing: tables→code-fence+post, MD→post, else→text
4. _build_outbound_payload — simplified to delegate

Result

• Tables render as code blocks (monospaced, visible)
• All other markdown (bold, italic, links, lists, headings) renders correctly
• No regression: 198/198 tests pass

Related

Fixes #21778

[asdlem]

Supersedes #22047. Both PRs fix the same regression (tables forcing plain-text mode), but this one goes further:

• fix(feishu): keep markdown tables in post payload so prose still renders #22047: removes the table-detection downgrade → tables become invisible (post md tag limitation)
• This PR: wraps tables in code fences + emits code_block tags → tables render as visible code blocks

Same bug fix + better UX for table content.

[asdlem]

Hi @teknium1 — gentle ping for review. This PR fixes #21778. Rebased cleanly onto main, 205 tests passing. Wraps tables in code blocks for Feishu rendering. Same pattern as #27852 → #28475. Thanks!

[asdlem]

main now has _MARKDOWN_TABLE_RE fallback to plain text — the white-screen bug is resolved. The code_block approach here is more elegant but functionally redundant.