feat(trace):LangGraph -> lead_agent and set custom agent_name to run_name (#3101)

* feat(trace):LangGraph -> lead_agent and set user custom agent name to run_name

* feat(trace):follow github copilot suggest

* feat(trace):Refactor run_name resolution and improve test coverage

backend/tests/test_run_naming.py

@@ -0,0 +1,34 @@
+from deerflow.runtime.runs.naming import resolve_root_run_name
+
+
+def test_resolve_root_run_name_from_context_agent_name():
+    assert resolve_root_run_name({"context": {"agent_name": "finalis"}}, "lead_agent") == "finalis"
+
+
+def test_resolve_root_run_name_from_configurable_agent_name():
+    assert resolve_root_run_name({"configurable": {"agent_name": "finalis"}}, "lead_agent") == "finalis"
+
+
+def test_resolve_root_run_name_falls_back_to_assistant_id():
+    assert resolve_root_run_name({}, "my-agent") == "my-agent"
+
+
+def test_resolve_root_run_name_falls_back_to_lead_agent():
+    assert resolve_root_run_name({}, None) == "lead_agent"
+
+
+def test_resolve_root_run_name_prefers_context_over_configurable():
+    config = {
+        "context": {"agent_name": "ctx-agent"},
+        "configurable": {"agent_name": "cfg-agent"},
+    }
+
+    assert resolve_root_run_name(config, "lead_agent") == "ctx-agent"
+
+
+def test_resolve_root_run_name_ignores_blank_agent_name():
+    assert resolve_root_run_name({"context": {"agent_name": "   "}}, "my-agent") == "my-agent"
+
+
+def test_resolve_root_run_name_ignores_non_string_agent_name():
+    assert resolve_root_run_name({"context": {"agent_name": None}}, "my-agent") == "my-agent"