feat(tracing): add optional Langfuse support (#1717)

* feat(tracing): add optional Langfuse support

* Fix tracing fail-fast behavior for explicitly enabled providers

* fix(lint)

backend/packages/harness/deerflow/tracing/factory.py

@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+from typing import Any
+
+from deerflow.config import (
+    get_enabled_tracing_providers,
+    get_tracing_config,
+    validate_enabled_tracing_providers,
+)
+
+
+def _create_langsmith_tracer(config) -> Any:
+    from langchain_core.tracers.langchain import LangChainTracer
+
+    return LangChainTracer(project_name=config.project)
+
+
+def _create_langfuse_handler(config) -> Any:
+    from langfuse import Langfuse
+    from langfuse.langchain import CallbackHandler as LangfuseCallbackHandler
+
+    # langfuse>=4 initializes project-specific credentials through the client
+    # singleton; the LangChain callback then attaches to that configured client.
+    Langfuse(
+        secret_key=config.secret_key,
+        public_key=config.public_key,
+        host=config.host,
+    )
+    return LangfuseCallbackHandler(public_key=config.public_key)
+
+
+def build_tracing_callbacks() -> list[Any]:
+    """Build callbacks for all explicitly enabled tracing providers."""
+    validate_enabled_tracing_providers()
+    enabled_providers = get_enabled_tracing_providers()
+    if not enabled_providers:
+        return []
+
+    tracing_config = get_tracing_config()
+    callbacks: list[Any] = []
+
+    for provider in enabled_providers:
+        if provider == "langsmith":
+            try:
+                callbacks.append(_create_langsmith_tracer(tracing_config.langsmith))
+            except Exception as exc:  # pragma: no cover - exercised via tests with monkeypatch
+                raise RuntimeError(f"LangSmith tracing initialization failed: {exc}") from exc
+        elif provider == "langfuse":
+            try:
+                callbacks.append(_create_langfuse_handler(tracing_config.langfuse))
+            except Exception as exc:  # pragma: no cover - exercised via tests with monkeypatch
+                raise RuntimeError(f"Langfuse tracing initialization failed: {exc}") from exc
+
+    return callbacks