[BUG] Slack MCP draft messages do not format lists, while sent messages do

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Slack connector: `slack_send_message_draft` flattens all list formatting; `slack_send_message` does not.
Any draft-for-human-review workflow loses list formatting, forcing manual re-formatting before sending.

- Feedback ID: 59cbd311-43de-4edc-8953-1a06f469ccfc

**Errors**
Draft renders literal "•" text characters (numbered lists also flatten). Passing a Block Kit {"blocks":[...]} payload as the message string renders as literal JSON.

### What Should Happen?

Draft should contain a native Slack bulleted list, like the sent message does.

### Steps to Reproduce

1. have a Slack MCP setup
2. Call `slack_send_message_draft` tool with markdown: `- item one\n- item two`. You can just ask Claude to "send `- item one\n- item two` to myself as a draft Slack message.
3. Look at Slack and note that the draft message only shows raw unicode bullets, not a real list. You can easily see it by editing the draft to add a real list below; the alignment isn't the same.

### Claude Model

Other

### Is this a regression?

No, this never worked

### Last Working Version

_No response_

### Claude Code Version

2.1.236

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

iTerm2

### Additional Information

_No response_

Start with the `slack_send_message_draft` MCP tool and reproduce the markdown list behavior described in the issue, then compare it with `slack_send_message`. Trace how each tool converts message content for Slack and verify that draft output preserves bulleted and numbered lists as native Slack formatting rather than literal bullets or JSON.

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