feat: add LangSmith tracing integration (#878)

* feat: add LangSmith tracing integration

Add optional LangSmith tracing support that can be enabled via environment
variables (LANGSMITH_TRACING, LANGSMITH_API_KEY, LANGSMITH_PROJECT,
LANGSMITH_ENDPOINT). When enabled, a LangChainTracer callback is attached
to chat models and run metadata is injected for trace tagging.

Co-Authored-By: Claude <noreply@anthropic.com>

* Update backend/src/config/tracing_config.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update backend/src/agents/lead_agent/agent.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update backend/src/agents/lead_agent/agent.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update backend/src/models/factory.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Add threading lock to ensure thread-safe access to tracing configuration

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/src/config/tracing_config.py

@@ -0,0 +1,51 @@
+import logging
+import os
+from pydantic import BaseModel, Field
+import threading
+
+logger = logging.getLogger(__name__)
+_config_lock = threading.Lock()
+
+class TracingConfig(BaseModel):
+    """Configuration for LangSmith tracing."""
+
+    enabled: bool = Field(...)
+    api_key: str | None = Field(...)
+    project: str = Field(...)
+    endpoint: str = Field(...)
+
+    @property
+    def is_configured(self) -> bool:
+        """Check if tracing is fully configured (enabled and has API key)."""
+        return self.enabled and bool(self.api_key)
+
+
+_tracing_config: TracingConfig | None = None
+
+
+def get_tracing_config() -> TracingConfig:
+    """Get the current tracing configuration from environment variables.
+    Returns:
+        TracingConfig with current settings.
+    """
+    global _tracing_config
+    if _tracing_config is not None:
+        return _tracing_config
+    with _config_lock:
+        if _tracing_config is not None:  # Double-check after acquiring lock
+            return _tracing_config
+        _tracing_config = TracingConfig(
+            enabled=os.environ.get("LANGSMITH_TRACING", "").lower() == "true",
+            api_key=os.environ.get("LANGSMITH_API_KEY"),
+            project=os.environ.get("LANGSMITH_PROJECT", "deer-flow"),
+            endpoint=os.environ.get("LANGSMITH_ENDPOINT", "https://api.smith.langchain.com"),
+        )
+        return _tracing_config
+    
+def is_tracing_enabled() -> bool:
+    """Check if LangSmith tracing is enabled and configured.
+    Returns:
+        True if tracing is enabled and has an API key.
+    """
+    return get_tracing_config().is_configured
+