Add spec for langchain based agent

Signed-off-by: Mihai Criveti <[email protected]>

Author: crivetimihai

agent_runtimes/langchain_agent/README.md

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+A configurable Langchain agent that supports MCP and integrates with the MCP Gateway via streamable HTTP + Auth.
+
+Tools can be specified as a CSV list.
+
+Exposes an OpenAI compatible API.
+
+Endpoints for:
+
+/health
+/ready
+/list_tools
+
+etc. are provided.