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>

tests/test_anthropic_adapter.py

@@ -801,6 +801,48 @@ class TestConvertMessages:
         assert all(not (b.get("type") == "text" and b.get("text") == "") for b in assistant_blocks)
         assert any(b.get("type") == "tool_use" for b in assistant_blocks)
 
+    def test_empty_user_message_string_gets_placeholder(self):
+        """Empty user message strings should get '(empty message)' placeholder.
+
+        Anthropic rejects requests with empty user message content.
+        Regression test for #3143 — Discord @mention-only messages.
+        """
+        messages = [
+            {"role": "user", "content": ""},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["role"] == "user"
+        assert result[0]["content"] == "(empty message)"
+
+    def test_whitespace_only_user_message_gets_placeholder(self):
+        """Whitespace-only user messages should also get placeholder."""
+        messages = [
+            {"role": "user", "content": "   \n\t  "},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["content"] == "(empty message)"
+
+    def test_empty_user_message_list_gets_placeholder(self):
+        """Empty content list for user messages should get placeholder block."""
+        messages = [
+            {"role": "user", "content": []},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["role"] == "user"
+        assert isinstance(result[0]["content"], list)
+        assert len(result[0]["content"]) == 1
+        assert result[0]["content"][0] == {"type": "text", "text": "(empty message)"}
+
+    def test_user_message_with_empty_text_blocks_gets_placeholder(self):
+        """User message with only empty text blocks should get placeholder."""
+        messages = [
+            {"role": "user", "content": [{"type": "text", "text": ""}, {"type": "text", "text": "  "}]},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["role"] == "user"
+        assert isinstance(result[0]["content"], list)
+        assert result[0]["content"] == [{"type": "text", "text": "(empty message)"}]
+
 
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