fix: validate empty user messages to prevent Anthropic API 400 errors (#3322)

When user messages have empty content (e.g., Discord @mention-only
messages, unrecognized attachments), the Anthropic API rejects the
request with 'user messages must have non-empty content'.

Changes:
- anthropic_adapter.py: Add empty content validation for user messages
  (string and list formats), matching the existing pattern for assistant
  and tool messages. Empty content gets '(empty message)' placeholder.

- discord.py: Defense-in-depth check at gateway layer to catch empty
  messages before they enter session history.

- Add 4 regression tests covering empty string, whitespace-only,
  empty list, and empty text block scenarios.

Fixes #3143

Co-authored-by: Bartok9 <bartok9@users.noreply.github.com>

gateway/platforms/discord.py

@@ -2096,6 +2096,11 @@ class DiscordAdapter(BasePlatformAdapter):
         if pending_text_injection:
             event_text = f"{pending_text_injection}\n\n{event_text}" if event_text else pending_text_injection
 
+        # Defense-in-depth: prevent empty user messages from entering session
+        # (can happen when user sends @mention-only with no other text)
+        if not event_text or not event_text.strip():
+            event_text = "(The user sent a message with no text content)"
+
         event = MessageEvent(
             text=event_text,
             message_type=msg_type,