Polish provider fallback and routing compat docs

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Co-authored-by: diasbruno <362368+diasbruno@users.noreply.github.com>

Author: Copilot

feature-flag/routing.lisp

@@ -44,7 +44,7 @@ Example:
                                         (request-data-key :feature-flag-evaluation-context))
   "Create a feature-flag client composed with request-scoped context.
 OBJECT-OF-INTEREST defaults to REQUEST and is used by RESOLVE-PROVIDER.
-NAME is accepted for backward compatibility and currently unused.
+NAME is accepted for backward compatibility; it was used by the removed CREATE-CLIENT API and is currently unused.
 Example:
   (client-for-request request :domain \"payments\" :evaluation-context '(:app \"checkout\"))
   => #<CLIENT ...>"

t/feature-flag-tests.lisp

@@ -94,7 +94,8 @@
 
 (defmethod resolve-provider ((holder provider-holder) domain)
   (or (and domain (cdr (assoc domain (provider-holder-domain-providers holder) :test #'equal)))
-      (provider-holder-default-provider holder)))
+      (provider-holder-default-provider holder)
+      (call-next-method)))
 
 (test provider-without-resolver-returns-default-and-provider-not-ready-error
   (let* ((client (acquire-client (make-instance 'provider :name "base")))