Standard message content

LangChain has introduced a new "Standard message content" feature to unify the diverse outputs from different LLM providers like OpenAI, Anthropic, and Google. This addresses the problem of diverging APIs by normalizing modern LLM capabilities such as reasoning, citations, and tool calls into a consistent format. All LangChain message objects now have a `.content_blocks` property that lazily computes this standard representation, allowing developers to build provider-agnostic applications more easily. The new standard content types are fully backward-compatible and will be available in LangChain 1.0 for both Python and JavaScript.