langchain: preserve result-level MCP `_meta` in tool results

The MCP specification allows a tool result to include result-level `_meta`, but `langchain.mcp` currently drops that field while converting the result into a LangChain tool message. This prevents applications from accessing out-of-band response metadata that the MCP SDK has already parsed. The previous `langchain-mcp-adapters` implementation received a proposed fix in [langchain-mcp-adapters#612](https://github.com/langchain-ai/langchain-mcp-adapters/pull/612), including tests for results containing `_meta` alone, structured content plus `_meta`, and neither field. Because `langchain.mcp` has a different result representation, the desired behavior should be implemented against the new API rather than porting that patch directly. A useful acceptance criterion would be that MCP result-level `_meta` remains available to downstream consumers without conflating it with the static `_meta` attached to a tool definition.