OptionalheadersRequest headers (instance headers merged with method headers)
HTTP method (GET, POST, etc.)
OptionalparamsURL parameters (extracted from url.searchParams for flat access)
Request path (original path without base URL)
OptionalpayloadRequest payload/body
OptionalstateInstance state
Fully-constructed URL including base URL, path, and query parameters.
Use url.pathname + url.search for cache/dedupe keys to exclude hash
while including the full path and query string.
Context object passed to request serializers for key generation.
Note: H and P represent the header/param type arguments, but the actual values are DictOrT and DictOrT
as stored by FetchEngine.