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/models/factory.py

@@ -1,8 +1,10 @@
+import logging
 from langchain.chat_models import BaseChatModel
 
-from src.config import get_app_config
+from src.config import get_app_config, get_tracing_config, is_tracing_enabled
 from src.reflection import resolve_class
 
+logger = logging.getLogger(__name__)
 
 def create_chat_model(name: str | None = None, thinking_enabled: bool = False, **kwargs) -> BaseChatModel:
     """Create a chat model instance from the config.
@@ -37,4 +39,20 @@ def create_chat_model(name: str | None = None, thinking_enabled: bool = False, *
             raise ValueError(f"Model {name} does not support thinking. Set `supports_thinking` to true in the `config.yaml` to enable thinking.") from None
         model_settings_from_config.update(model_config.when_thinking_enabled)
     model_instance = model_class(**kwargs, **model_settings_from_config)
+
+    if is_tracing_enabled():
+        try:
+            from langchain_core.tracers.langchain import LangChainTracer
+
+            tracing_config = get_tracing_config()
+            tracer = LangChainTracer(
+                project_name=tracing_config.project,
+            )
+            existing_callbacks = model_instance.callbacks or []
+            model_instance.callbacks = [*existing_callbacks, tracer]
+            logger.debug(
+                f"LangSmith tracing attached to model '{name}' (project='{tracing_config.project}')"
+            )
+        except Exception as e:
+            logger.warning(f"Failed to attach LangSmith tracing to model '{name}': {e}")
     return model_instance