Fix install page context, make the install page tests really test (#24858)

Fix #24856

Rename "context.contextKey" to "context.WebContextKey", this context is
for web context only. But the Context itself is not renamed, otherwise
it would cause a lot of changes (if we really want to rename it, there
could be a separate PR).

The old test code doesn't really test, the "install page" gets broken
not only one time, so use new test code to make sure the "install page"
could work.

modules/web/handler.go

@@ -22,7 +22,7 @@ type ResponseStatusProvider interface {
 // TODO: decouple this from the context package, let the context package register these providers
 var argTypeProvider = map[reflect.Type]func(req *http.Request) ResponseStatusProvider{
 	reflect.TypeOf(&context.APIContext{}):     func(req *http.Request) ResponseStatusProvider { return context.GetAPIContext(req) },
-	reflect.TypeOf(&context.Context{}):        func(req *http.Request) ResponseStatusProvider { return context.GetContext(req) },
+	reflect.TypeOf(&context.Context{}):        func(req *http.Request) ResponseStatusProvider { return context.GetWebContext(req) },
 	reflect.TypeOf(&context.PrivateContext{}): func(req *http.Request) ResponseStatusProvider { return context.GetPrivateContext(req) },
 }