fix: improve WhatsApp UX — chunking, formatting, streaming (#8723)

Three changes that address the poor WhatsApp experience reported by users:

1. Reclassify WhatsApp from TIER_LOW to TIER_MEDIUM in display_config.py
   — enables streaming and tool progress via the existing Baileys /edit
   bridge endpoint. Users now see progressive responses instead of
   minutes of silence followed by a wall of text.

2. Lower MAX_MESSAGE_LENGTH from 65536 to 4096 and add proper chunking
   — send() now calls format_message() and truncate_message() before
   sending, then loops through chunks with a small delay between them.
   The base class truncate_message() already handles code block boundary
   detection (closes/reopens fences at chunk boundaries). reply_to is
   only set on the first chunk.

3. Override format_message() with WhatsApp-specific markdown conversion
   — converts **bold** to *bold*, ~~strike~~ to ~strike~, headers to
   bold text, and [links](url) to text (url). Code blocks and inline
   code are protected from conversion via placeholder substitution.

Together these fix the two user complaints:
- 'sends the whole code all the time' → now chunked at 4K with proper
  formatting
- 'terminal gets interrupted and gets cooked' → streaming + tool progress
  give visual feedback so users don't accidentally interrupt with
  follow-up messages

tests/gateway/test_display_config.py

@@ -189,14 +189,14 @@ class TestPlatformDefaults:
         """Slack, Mattermost, Matrix default to 'new' tool progress."""
         from gateway.display_config import resolve_display_setting
 
-        for plat in ("slack", "mattermost", "matrix", "feishu"):
+        for plat in ("slack", "mattermost", "matrix", "feishu", "whatsapp"):
             assert resolve_display_setting({}, plat, "tool_progress") == "new", plat
 
     def test_low_tier_platforms(self):
-        """Signal, WhatsApp, etc. default to 'off' tool progress."""
+        """Signal, BlueBubbles, etc. default to 'off' tool progress."""
         from gateway.display_config import resolve_display_setting
 
-        for plat in ("signal", "whatsapp", "bluebubbles", "weixin", "wecom", "dingtalk"):
+        for plat in ("signal", "bluebubbles", "weixin", "wecom", "dingtalk"):
             assert resolve_display_setting({}, plat, "tool_progress") == "off", plat
 
     def test_minimal_tier_platforms(self):