v4.40.0

• fix(google-genai): break circular reference in span event body by @dmontagu in #2157
• Remove logfire.variables_clear since it's for tests, not users by @alexmojaki in #2033
• Fix three SSE reliability gaps in LogfireRemoteVariableProvider by @dmontagu in #2144
• Log transient variables-refresh failures as warnings, not errors by @dmontagu in #2111
• feat(variables): ETag/conditional GET and silent 304 handling by @dmontagu in #2146
• Fix metric resource PID after fork by @alexmojaki in #2189