Prevent false green validation results when MCP build streams end early

validate_flow already tracked how many nodes were expected, but the final
success path only checked whether a failing event had been seen. An early
end event could therefore return valid=true even when only part of the
flow finished. This change treats that condition as invalid and records a
flow-level error, while keeping the existing fast-fail behavior unchanged.

Constraint: Keep the fix narrowly scoped to validate_flow rather than folding it into broader MCP/session work
Rejected: Wait for PR langflow-ai#12528 to land | it does not close this exact success-path gap on current main
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If validate_flow semantics change again, preserve the invariant that success means every expected component completed
Tested: cd src/lfx && uv sync && uv run pytest tests/unit/mcp/test_validate_flow.py -q
Not-tested: End-to-end MCP validation against a live Langflow server
Related: langflow-ai#12757

Author: shaun0927

src/lfx/src/lfx/mcp/server.py

@@ -1145,6 +1145,18 @@ async def validate_flow(flow_id: str) -> dict[str, Any]:
             "errors": errors,
         }
 
+    if len(completed) != expected:
+        return {
+            "valid": False,
+            "component_count": len(completed),
+            "errors": [
+                {
+                    "component_id": "flow",
+                    "error": f"Build ended early: {len(completed)}/{expected} components completed",
+                }
+            ],
+        }
+
     return {
         "valid": True,
         "component_count": len(completed),

src/lfx/tests/unit/mcp/test_validate_flow.py

@@ -162,3 +162,21 @@ async def _stream(*_args, **_kwargs):
         assert result["valid"] is False
         assert "Build request failed" in result["error"]
         assert result["component_count"] == 0
+
+    async def test_end_event_before_all_components_finish_returns_failure(self) -> None:
+        """An early end event must not be reported as a valid full build."""
+        from lfx.mcp.server import validate_flow
+
+        flow = _flow_with_nodes(2)
+        events = [
+            _end_vertex_event("A-1", valid=True),
+            _end_event(),
+        ]
+        client = _stream_client(events)
+
+        with _patch_validate(flow=flow, client=client):
+            result = await validate_flow("flow-1")
+
+        assert result["valid"] is False
+        assert result["component_count"] == 1
+        assert result["errors"] == [{"component_id": "flow", "error": "Build ended early: 1/2 components completed"}]