Function default_user_agent 

pub fn default_user_agent() -> HeaderValue

The default User-Agent value is defect-http/{CARGO_PKG_VERSION} ({DEFECT_HTTP_GIT_SHA}).

DEFECT_HTTP_GIT_SHA is injected by build.rs: it first reads the build-time environment variable DEFECT_HTTP_BUILD_SHA (for downstream packaging scenarios without a .git directory), then falls back to running git rev-parse, and finally degrades to "unknown" if neither is available.