fix(mcp): accept transport field as alias for type (#3238) (#3243)

The official MCP configuration schema uses `transport` to specify the
transport mechanism (stdio/sse/http), but `McpServerConfig` only honored
`type` and defaulted to `stdio`. Remote MCP servers configured with just
`transport: sse` were therefore misidentified as stdio and failed with
"with stdio transport requires 'command' field".

Add a model validator that promotes `transport` to `type` when only
`transport` is provided, while keeping `type` authoritative when both
are set. This matches the MCP-spec field name without breaking existing
configurations.

Fixes #3238

backend/tests/test_mcp_client_config.py

@@ -83,6 +83,41 @@ def test_build_server_params_rejects_unsupported_transport():
         build_server_params("bad-transport", config)
 
 
+@pytest.mark.parametrize("transport", ["sse", "http"])
+def test_mcp_server_config_accepts_transport_alias(transport: str):
+    """The MCP-spec ``transport`` field should be accepted as an alias for ``type``.
+
+    Regression test for https://github.com/bytedance/deer-flow/issues/3238 — a
+    remote MCP server configured with only ``transport: sse`` was previously
+    misidentified as ``stdio`` (the default for ``type``).
+    """
+    config = McpServerConfig.model_validate(
+        {
+            "transport": transport,
+            "url": "https://example.com/mcp",
+        }
+    )
+
+    assert config.type == transport
+
+    params = build_server_params("aliased-server", config)
+    assert params["transport"] == transport
+    assert params["url"] == "https://example.com/mcp"
+
+
+def test_mcp_server_config_type_takes_precedence_over_transport():
+    """When both ``type`` and ``transport`` are provided, ``type`` wins."""
+    config = McpServerConfig.model_validate(
+        {
+            "type": "http",
+            "transport": "sse",
+            "url": "https://example.com/mcp",
+        }
+    )
+
+    assert config.type == "http"
+
+
 def test_build_servers_config_returns_empty_when_no_enabled_servers():
     extensions = ExtensionsConfig(
         mcp_servers={