s/mcp_server_requirements/mcp_servers (#9212)

A simple `s/mcp_server_requirements/mcp_servers/g` for an unreleased
feature. @bolinfest correctly pointed out, it's already in
`requirements.toml` so the `_requirements` is redundant.

codex-rs/app-server/src/config_api.rs

@@ -135,7 +135,7 @@ mod tests {
                 CoreSandboxModeRequirement::ReadOnly,
                 CoreSandboxModeRequirement::ExternalSandbox,
             ]),
-            mcp_server_requirements: None,
+            mcp_servers: None,
         };
 
         let mapped = map_requirements_toml_to_api(requirements);