Add integration and edge-case coverage for the new
_extract_forwarded_headers_from_config flow: wrapper-dict and raw x-*
sources, context > configurable precedence, mixed-case key normalization,
RuntimeError early-return clears stale ContextVar, exception path clears
stale ContextVar, None and empty-config fallbacks, and a sync/async
parity check that both call paths run the extraction.