feat(gateway): render terminal tool calls as native bash code blocks on markdown platforms (#41215)

Tool-progress now shows a terminal command in a ```bash fenced block —
full command, no surrounding quotes, no label, no 40-char truncation —
instead of the noisy `terminal: "cmd…"` line, on every platform that
renders markdown code blocks (Telegram, Slack, Matrix, WhatsApp, Feishu,
Weixin, Discord). Plain-text platforms keep the compact preview line.

Gated on a new `BasePlatformAdapter.supports_code_blocks` capability
(default False) rather than a hardcoded platform list, so plugin adapters
(Discord lives in plugins/platforms/) opt in by setting the flag. Applies
to both all/new and verbose progress modes, with a safe fallback when the
command arg is missing or blank.

plugins/platforms/discord/adapter.py

@@ -573,6 +573,7 @@ class DiscordAdapter(BasePlatformAdapter):
     # Discord message limits
     MAX_MESSAGE_LENGTH = 2000
     _SPLIT_THRESHOLD = 1900  # near the 2000-char split point
+    supports_code_blocks = True  # Discord markdown renders fenced code blocks natively
 
     # Auto-disconnect from voice channel after this many seconds of inactivity
     VOICE_TIMEOUT = 300