fix: honor MCP tool error results

tests/tools/test_mcp_tool.py

@@ -19,7 +19,7 @@ from nanobot.agent.tools.mcp import (
     _sanitize_name,
     connect_mcp_servers,
 )
-from nanobot.agent.tools.registry import ToolRegistry
+from nanobot.agent.tools.registry import ToolRegistry, is_tool_error_result
 from nanobot.config.schema import MCPServerConfig
 
 
@@ -304,6 +304,38 @@ async def test_execute_returns_text_blocks() -> None:
     assert result == "hello\n42"
 
 
+@pytest.mark.asyncio
+async def test_execute_wraps_mcp_is_error_result() -> None:
+    async def call_tool(_name: str, arguments: dict) -> object:
+        return SimpleNamespace(
+            content=[_FakeTextContent("Error: server-side MCP failure")],
+            isError=True,
+        )
+
+    wrapper = _make_wrapper(SimpleNamespace(call_tool=call_tool))
+
+    result = await wrapper.execute()
+
+    assert result == "Error: server-side MCP failure"
+    assert is_tool_error_result(wrapper.name, result)
+
+
+@pytest.mark.asyncio
+async def test_execute_preserves_success_text_that_starts_with_error() -> None:
+    async def call_tool(_name: str, arguments: dict) -> object:
+        return SimpleNamespace(
+            content=[_FakeTextContent("Error: generated report successfully")],
+            isError=False,
+        )
+
+    wrapper = _make_wrapper(SimpleNamespace(call_tool=call_tool))
+
+    result = await wrapper.execute()
+
+    assert result == "Error: generated report successfully"
+    assert not is_tool_error_result(wrapper.name, result)
+
+
 # Smallest valid 1x1 PNG, base64 without the data: prefix.
 _PNG_B64 = (
     "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mP8"