OpenAI Agents SDK 0.20.0 switches the default model to gpt-5.6-luna and takes a breaking MCP v2 dependency

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Released early on Aug 11, the minor bump changes the implicit default model to gpt-5.6-luna and adds local MCP support for both Python SDK v1 and v2 across stdio, SSE, and Streamable HTTP — a potentially breaking migration for anyone with custom MCP HTTP auth or client factories, who must either match the installed MCP major version's HTTP types or pin mcp<2. The release also adds RunState.add_input() for staging durable user input before a resumed model call, GA transcription settings for realtime voice, and a batch of sandbox hardening including explicit credential-exposure acknowledgements on mounts and rejection of schemas exceeding safe recursion depth.