Follow-up to the try/except guards added in the previous commit.
Four sibling call sites all read HERMES_AGENT_TIMEOUT /
HERMES_AGENT_TIMEOUT_WARNING / HERMES_AGENT_NOTIFY_INTERVAL via the
same read-env-or-fallback pattern, so factor it into _float_env(name,
default) alongside the existing _auto_continue_freshness_window()
helper.