Introducing LangSmith Trajectories: A readable view of every agent session

Introduces 'Trajectories' in LangSmith, a new chronological, conversational view of agent sessions. Trajectories aggregate messages from humans, AI, and tools across the main agent and subagents, presenting them in their order of appearance. This feature aims to improve root cause analysis by providing a readable path of agent behavior, reduce friction for expert feedback by offering a less technical view than raw traces, and enable continuous improvement by allowing scoring with online evaluators and saving sessions to datasets for fine-tuning.