fix(slack): comprehensive mrkdwn formatting — 6 bug fixes + 52 tests

Fixes blockquote > escaping, edit_message raw markdown, ***bold italic***
handling, HTML entity double-escaping (&), Wikipedia URL parens
truncation, and step numbering format. Also adds format_message to the
tool-layer _send_to_platform for consistent formatting across all
delivery paths.

Changes:
- Protect Slack entities (<@user>, <https://...|label>, <!here>) from
  escaping passes
- Protect blockquote > markers before HTML entity escaping
- Unescape-before-escape for idempotent HTML entity handling
- ***bold italic*** → *_text_* conversion (before **bold** pass)
- URL regex upgraded to handle balanced parentheses
- mrkdwn:True flag on chat_postMessage payloads
- format_message applied in edit_message and send_message_tool
- 52 new tests (format, edit, streaming, splitting, tool chunking)
- Use reversed(dict) idiom for placeholder restoration

Based on PR #3715 by dashed, cherry-picked onto current main.

tools/send_message_tool.py

@@ -322,6 +322,13 @@ async def _send_to_platform(platform, pconfig, chat_id, message, thread_id=None,
 
     media_files = media_files or []
 
+    if platform == Platform.SLACK and message:
+        try:
+            slack_adapter = SlackAdapter.__new__(SlackAdapter)
+            message = slack_adapter.format_message(message)
+        except Exception:
+            logger.debug("Failed to apply Slack mrkdwn formatting in _send_to_platform", exc_info=True)
+
     # Platform message length limits (from adapter class attributes)
     _MAX_LENGTHS = {
         Platform.TELEGRAM: TelegramAdapter.MAX_MESSAGE_LENGTH,
@@ -571,7 +578,8 @@ async def _send_slack(token, chat_id, message):
         url = "https://slack.com/api/chat.postMessage"
         headers = {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}
         async with aiohttp.ClientSession(timeout=aiohttp.ClientTimeout(total=30)) as session:
-            async with session.post(url, headers=headers, json={"channel": chat_id, "text": message}) as resp:
+            payload = {"channel": chat_id, "text": message, "mrkdwn": True}
+            async with session.post(url, headers=headers, json=payload) as resp:
                 data = await resp.json()
                 if data.get("ok"):
                     return {"success": True, "platform": "slack", "chat_id": chat_id, "message_id": data.get("ts")}