NestBot: Fix ** bold markdown formatting for Slack responses (ref #2663)

## Describe the bug

Slack uses single asterisk markdown (`*text*`) for bold formatting.
However, NestBot sometimes returns responses containing double asterisks (`**text**`).

Slack does not render `**bold**` correctly, so users see the literal `**` characters instead of bold text.

This results in:
- broken formatting
- poor readability
- unpolished bot responses

## To Reproduce

Steps to reproduce the behavior:

1. Ask NestBot any question
2. Wait for an AI response that contains formatted text
3. If the message includes `**bold text**`, Slack displays the literal `**` characters
4. Bold formatting does not render

Example output:
Here are the important steps to follow

Displayed in Slack as:
Here are the **important** steps to follow
(instead of bold text)

## Expected behavior
NestBot responses should follow Slack markdown rules.
Bold text should render correctly using:

**instead of:**
All AI-generated responses should automatically convert markdown into Slack-compatible formatting before posting.

## Are you going to work on fixing this?

- [x] Yes
- [ ] No

## Additional context
This appears to happen because standard Markdown (`**bold**`) is passed directly to Slack without conversion.
Slack formatting rules differ slightly and require single asterisks.

### Proposed fix
Normalize markdown before sending messages to Slack.

Suggested location:
`backend/apps/slack/utils.py
`

Refernce #2663

Start in backend/apps/slack/utils.py, the suggested location for normalizing responses before they are sent to Slack. Reproduce a response containing **bold text**, then verify that the posted message uses single asterisks and no longer displays literal double-asterisk markers.

Written by the indexing model from the issue text.