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>

agent/anthropic_adapter.py

@@ -706,14 +706,21 @@ def convert_messages_to_anthropic(
                 result.append({"role": "user", "content": [tool_result]})
             continue
 
-        # Regular user message
+        # Regular user message — validate non-empty content (Anthropic rejects empty)
         if isinstance(content, list):
             converted_blocks = _convert_content_to_anthropic(content)
-            result.append({
-                "role": "user",
-                "content": converted_blocks or [{"type": "text", "text": ""}],
-            })
+            # Check if all text blocks are empty
+            if not converted_blocks or all(
+                b.get("text", "").strip() == ""
+                for b in converted_blocks
+                if isinstance(b, dict) and b.get("type") == "text"
+            ):
+                converted_blocks = [{"type": "text", "text": "(empty message)"}]
+            result.append({"role": "user", "content": converted_blocks})
         else:
+            # Validate string content is non-empty
+            if not content or (isinstance(content, str) and not content.strip()):
+                content = "(empty message)"
             result.append({"role": "user", "content": content})
 
     # Strip orphaned tool_use blocks (no matching tool_result follows)