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

gateway/display_config.py

@@ -82,7 +82,7 @@ _PLATFORM_DEFAULTS: dict[str, dict[str, Any]] = {
 
     # Tier 3 — no edit support, progress messages are permanent
     "signal":          _TIER_LOW,
-    "whatsapp":        _TIER_LOW,
+    "whatsapp":        _TIER_MEDIUM,  # Baileys bridge supports /edit
     "bluebubbles":     _TIER_LOW,
     "weixin":          _TIER_LOW,
     "wecom":           _TIER_LOW,