5.18.0

(feat): Add opt-in support for a structured User-Agent header of the form {sdkName}/{version} ({os}; {arch}) Python/{pythonVersion}, matching the Twilio helper-library shape. This is gated behind the include_platform_headers config option and is disabled by default. When enabled, the platform + runtime information is consolidated into the User-Agent header, superseding the default {package}/{version} value, and the discrete X-Fern-Runtime / X-Fern-Platform headers are no longer emitted. The platform group and runtime segment are dropped gracefully when they cannot be determined, and the header remains subject to omit_fern_headers.