fix: use camelCase structuredContent attr, prefer structured over text

- The MCP SDK Pydantic model uses camelCase (structuredContent), not
  snake_case (structured_content). The original getattr was a silent no-op.
- When structuredContent is present, return it AS the result instead of
  alongside text — the structured payload is the machine-readable data.
- Move test file to tests/tools/ and fix fake class to use camelCase.
- Patch _run_on_mcp_loop in tests so the handler actually executes.

tests/tools/test_mcp_structured_content.py

@@ -0,0 +1,111 @@
+"""Tests for MCP tool structuredContent preservation."""
+
+import asyncio
+import json
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from tools import mcp_tool
+
+
+class _FakeContentBlock:
+    """Minimal content block with .text and .type attributes."""
+
+    def __init__(self, text: str, block_type: str = "text"):
+        self.text = text
+        self.type = block_type
+
+
+class _FakeCallToolResult:
+    """Minimal CallToolResult stand-in.
+
+    Uses camelCase ``structuredContent`` / ``isError`` to match the real
+    MCP SDK Pydantic model (``mcp.types.CallToolResult``).
+    """
+
+    def __init__(self, content, is_error=False, structuredContent=None):
+        self.content = content
+        self.isError = is_error
+        self.structuredContent = structuredContent
+
+
+def _fake_run_on_mcp_loop(coro, timeout=30):
+    """Run an MCP coroutine directly in a fresh event loop."""
+    loop = asyncio.new_event_loop()
+    try:
+        return loop.run_until_complete(coro)
+    finally:
+        loop.close()
+
+
+@pytest.fixture
+def _patch_mcp_server():
+    """Patch _servers and the MCP event loop so _make_tool_handler can run."""
+    fake_session = MagicMock()
+    fake_server = SimpleNamespace(session=fake_session)
+    with patch.dict(mcp_tool._servers, {"test-server": fake_server}), \
+         patch("tools.mcp_tool._run_on_mcp_loop", side_effect=_fake_run_on_mcp_loop):
+        yield fake_session
+
+
+class TestStructuredContentPreservation:
+    """Ensure structuredContent from CallToolResult is forwarded."""
+
+    def test_text_only_result(self, _patch_mcp_server):
+        """When no structuredContent, result is text-only (existing behaviour)."""
+        session = _patch_mcp_server
+        session.call_tool = AsyncMock(
+            return_value=_FakeCallToolResult(
+                content=[_FakeContentBlock("hello")],
+            )
+        )
+        handler = mcp_tool._make_tool_handler("test-server", "my-tool", 30.0)
+        raw = handler({})
+        data = json.loads(raw)
+        assert data == {"result": "hello"}
+
+    def test_structured_content_is_the_result(self, _patch_mcp_server):
+        """When structuredContent is present, it becomes the result directly."""
+        session = _patch_mcp_server
+        payload = {"value": "secret-123", "revealed": True}
+        session.call_tool = AsyncMock(
+            return_value=_FakeCallToolResult(
+                content=[_FakeContentBlock("OK")],
+                structuredContent=payload,
+            )
+        )
+        handler = mcp_tool._make_tool_handler("test-server", "my-tool", 30.0)
+        raw = handler({})
+        data = json.loads(raw)
+        assert data["result"] == payload
+
+    def test_structured_content_none_falls_back_to_text(self, _patch_mcp_server):
+        """When structuredContent is explicitly None, fall back to text."""
+        session = _patch_mcp_server
+        session.call_tool = AsyncMock(
+            return_value=_FakeCallToolResult(
+                content=[_FakeContentBlock("done")],
+                structuredContent=None,
+            )
+        )
+        handler = mcp_tool._make_tool_handler("test-server", "my-tool", 30.0)
+        raw = handler({})
+        data = json.loads(raw)
+        assert data == {"result": "done"}
+
+    def test_empty_text_with_structured_content(self, _patch_mcp_server):
+        """When content blocks are empty but structuredContent exists."""
+        session = _patch_mcp_server
+        payload = {"status": "ok", "data": [1, 2, 3]}
+        session.call_tool = AsyncMock(
+            return_value=_FakeCallToolResult(
+                content=[],
+                structuredContent=payload,
+            )
+        )
+        handler = mcp_tool._make_tool_handler("test-server", "my-tool", 30.0)
+        raw = handler({})
+        data = json.loads(raw)
+        assert data["result"] == payload