Invalidate a cached route without hand-building a Request

[allen0099]

Problem

Invalidating a cached GET route from the POST/PUT handler that changes the data needs a hand-built request. The documented recipe copies the ASGI scope and rewrites method, path and query_string:

scope = dict(request.scope)
scope["method"] = "GET"
scope["path"] = f"/items/{item_id}"
scope["query_string"] = b""
await invalidate(StarletteRequest(scope))

It is the most awkward snippet in the docs. The obvious call, invalidate(request) with the PUT request itself, quietly returns False, because it builds the PUT key.

Proposal

Add a helper that builds the key without a hand-made request. Two options:

• await invalidate_path("/items/1", request=request, query=...), which takes host and scheme from the current request and uses GET;
• a method on the decorated route, e.g. await read_item.invalidate(item_id=1).

Either way it must honour key_builder and vary, as invalidate() does. It should also say clearly (warn or document) that invalidate() uses the method of the request it is given.