fix: ensure output of codex-rs/mcp-types/generate_mcp_types.py matches codex-rs/mcp-types/src/lib.rs

[bolinfest]

#3395 updated mcp-types/src/lib.rs by hand, but that file is generated code that is produced by mcp-types/generate_mcp_types.py. Unfortunately, we do not have anything in CI to verify this right now, but I will address that in a subsequent PR.

#3395 ended up introducing a change that added a required field when deserializing InitializeResult, breaking Codex when used as an MCP client, so the quick fix in #3436 was to make the new field Optional with skip_serializing_if = "Option::is_none", but that did not address the problem that mcp-types/generate_mcp_types.py and mcp-types/src/lib.rs are out of sync.

This PR gets things back to where they are in sync. It removes the custom mcp_types::McpClientInfo type that was added to mcp-types/src/lib.rs and forces us to use the generated mcp_types::Implementation type. Though this PR also updates generate_mcp_types.py to generate the additional user_agent: Optional<String> field on Implementation so that we can continue to specify it when Codex operates as an MCP server.

However, this also requires us to specify user_agent: None when Codex operates as an MCP client.

We may want to introduce our own InitializeResult type that is specific to when we run as a server to avoid this in the future, but my immediate goal is just to get things back in sync.

[gpeal]

Thank you 🙏