fix(provider): filter empty text content blocks causing API 400

When MCP tools return empty content, messages may contain empty-string
text blocks. OpenAI-compatible providers reject these with HTTP 400.

Changes:
- Add _prevent_empty_text_blocks() to filter empty text items from
  content lists and handle empty string content
- For assistant messages with tool_calls, set content to None (valid)
- For other messages, replace with '(empty)' placeholder
- Only copy message dict when modification is needed (zero-copy path
  for normal messages)

Co-Authored-By: nanobot <noreply@anthropic.com>

nanobot/providers/custom_provider.py

@@ -19,8 +19,12 @@ class CustomProvider(LLMProvider):
 
     async def chat(self, messages: list[dict[str, Any]], tools: list[dict[str, Any]] | None = None,
                    model: str | None = None, max_tokens: int = 4096, temperature: float = 0.7) -> LLMResponse:
-        kwargs: dict[str, Any] = {"model": model or self.default_model, "messages": messages,
-                                  "max_tokens": max(1, max_tokens), "temperature": temperature}
+        kwargs: dict[str, Any] = {
+            "model": model or self.default_model,
+            "messages": self._prevent_empty_text_blocks(messages),
+            "max_tokens": max(1, max_tokens),
+            "temperature": temperature,
+        }
         if tools:
             kwargs.update(tools=tools, tool_choice="auto")
         try:
@@ -45,3 +49,47 @@ class CustomProvider(LLMProvider):
 
     def get_default_model(self) -> str:
         return self.default_model
+
+    @staticmethod
+    def _prevent_empty_text_blocks(messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
+        """Filter empty text content blocks that cause provider 400 errors.
+
+        When MCP tools return empty content, messages may contain empty-string
+        text blocks. Most providers (OpenAI-compatible) reject these with 400.
+        This method filters them out before sending to the API.
+        """
+        patched: list[dict[str, Any]] = []
+        for msg in messages:
+            content = msg.get("content")
+
+            # Empty string content
+            if isinstance(content, str) and content == "":
+                clean = dict(msg)
+                if msg.get("role") == "assistant" and msg.get("tool_calls"):
+                    clean["content"] = None
+                else:
+                    clean["content"] = "(empty)"
+                patched.append(clean)
+                continue
+
+            # List content — filter out empty text items
+            if isinstance(content, list):
+                filtered = [
+                    item for item in content
+                    if not (isinstance(item, dict)
+                            and item.get("type") in {"text", "input_text", "output_text"}
+                            and item.get("text") == "")
+                ]
+                if filtered != content:
+                    clean = dict(msg)
+                    if filtered:
+                        clean["content"] = filtered
+                    elif msg.get("role") == "assistant" and msg.get("tool_calls"):
+                        clean["content"] = None
+                    else:
+                        clean["content"] = "(empty)"
+                    patched.append(clean)
+                    continue
+
+            patched.append(msg)
+        return patched