fix(gateway): WhatsApp/Signal hints affirm markdown instead of forbidding it (#53564)

The 'whatsapp' and 'signal' PLATFORM_HINTS told the agent 'Please do not
use markdown as it does not render' — factually wrong. Both adapters
actively convert markdown to native formatting:

- whatsapp_common.format_message(): **bold**, ~~strike~~, # headers,
  links, code blocks -> WhatsApp native syntax
- signal_format.markdown_to_signal(): same conversions via bodyRanges,
  plus '- item' / '* item' bullets -> '• ' Unicode bullets

The wrong hint made the agent strip bullets and bold the adapter would
have rendered (#12224). Rewrote both hints to mirror whatsapp_cloud:
markdown is auto-converted, bullet lists work, tables are not supported.
Added a contract test asserting markdown-converting platforms never
forbid markdown in their hint.

agent/prompt_builder.py

@@ -617,7 +617,12 @@ DEVELOPER_ROLE_MODELS = ("gpt-5", "codex")
 PLATFORM_HINTS = {
     "whatsapp": (
         "You are on a text messaging communication platform, WhatsApp. "
-        "Please do not use markdown as it does not render. "
+        "Standard markdown (**bold**, *italic*, ~~strike~~, # headers, "
+        "`code`, ```code blocks```, [links](url)) is auto-converted to "
+        "WhatsApp's native syntax (*bold*, _italic_, ~strike~, monospace) — "
+        "feel free to write in markdown, and use bullet lists ('- item') "
+        "freely. Tables are NOT supported — prefer bullet lists or labeled "
+        "key:value pairs. "
         "You can send media files natively: to deliver a file to the user, "
         "include MEDIA:/absolute/path/to/file in your response. The file "
         "will be sent as a native WhatsApp attachment — images (.jpg, .png, "
@@ -682,7 +687,11 @@ PLATFORM_HINTS = {
     ),
     "signal": (
         "You are on a text messaging communication platform, Signal. "
-        "Please do not use markdown as it does not render. "
+        "Standard markdown (**bold**, *italic*, ~~strike~~, # headers, "
+        "`code`, ```code blocks```) is auto-converted to Signal's native "
+        "rich formatting — feel free to write in markdown, and use bullet "
+        "lists ('- item') freely (they render as • bullets). Tables are NOT "
+        "supported — prefer bullet lists or labeled key:value pairs. "
         "You can send media files natively: to deliver a file to the user, "
         "include MEDIA:/absolute/path/to/file in your response. Images "
         "(.png, .jpg, .webp) appear as photos, audio as attachments, and other "

tests/agent/test_prompt_builder.py

@@ -1018,6 +1018,19 @@ class TestPromptBuilderConstants:
         hint = PLATFORM_HINTS["whatsapp_cloud"]
         assert "MEDIA:" in hint
 
+    def test_markdown_converting_platform_hints_do_not_forbid_markdown(self):
+        """#12224 — WhatsApp (Baileys) and Signal adapters actively convert
+        markdown to native formatting (gateway/platforms/whatsapp_common.py
+        format_message + signal_format.markdown_to_signal: bold, italic,
+        strikethrough, headers, bullets). Their hints previously told the
+        agent "do not use markdown", which made it strip bullets/bold the
+        adapter would have rendered. The hint must affirm markdown, not
+        forbid it."""
+        for key in ("whatsapp", "signal"):
+            hint = PLATFORM_HINTS[key]
+            assert "do not use markdown" not in hint.lower()
+            assert "markdown" in hint.lower()
+
     def test_cli_hint_does_not_suggest_media_tags(self):
         # Regression: MEDIA:/path tags are intercepted only by messaging
         # gateway platforms. On the CLI they render as literal text and