Bug Description

When Hermes sends multi-line code blocks to Feishu via the post message type with md tag, the code block is rendered collapsed and cannot be expanded by the user. Only the first ~6 lines are visible.

Steps to Reproduce

1. Configure Hermes with Feishu platform enabled (WebSocket mode)
2. Ask the agent to generate any response containing a multi-line code block (e.g., a Python function with 10+ lines)
3. Send the response to Feishu

Actual Behavior

The code block appears truncated after ~6 lines with no way to expand or view the full content. The message is delivered as a Feishu post message with md tag, which does not support interactive code block expansion.

Expected Behavior

Users should be able to click/expand the code block to view the full content, similar to how code blocks render in other messaging platforms or how Feishu's own code block component behaves.

Environment

• Hermes version: current main branch
• Platform: Feishu (WebSocket connection)
• Feishu client: desktop and mobile (both affected)

Relevant Context

• Feishu's md tag in post messages does not support interactive elements
• The gateway/platforms/feishu.py _build_markdown_post_payload() renders code blocks as static text
• Related: Issue [Feishu Bug] Markdown source code displayed instead of rendered card format #7022 mentions code block collapse but focuses on raw markdown display, not the inability to expand
• Issue [Feishu] Markdown tables not rendering in Feishu messages #9549 tracks markdown table rendering (separate but related Feishu md tag limitation)

Root Cause

The _build_markdown_post_payload() method in gateway/platforms/feishu.py wraps all markdown content (including code blocks) in a Feishu md tag. Feishu's md tag does not support interactive code block elements — code blocks are rendered as static, non-expandable text and get truncated after a few lines.

Suggested Fix

Route code block content to a Feishu Interactive Card (msg_type: interactive) with a tag: markdown element instead of the post type. CardKit 2.0's markdown element supports code fences that render with a "Copy" button and scrollable view in Feishu.

Alternatively, use Feishu's native tag: code element within card format for code-specific rendering with proper expansion support.