oc-basic: Parse @string entries in BiBTeX bibliographies

* lisp/oc-basic.el (org-cite-basic--parse-bibtex): Expand string
abbreviations in the .bib bibliography files.

Fixes https://list.orgmode.org/orgmode/25288.4658.491916.790389@gargle.gargle.HOWL/

lisp/oc-basic.el

@@ -240,7 +240,11 @@ Return a hash table with citation references as keys and fields alist as values.
                       (cons
                        (intern (downcase field))
                        (replace-regexp-in-string "[ \t\n]+" " " value)))))
-                 (bibtex-parse-entry t))
+                 ;; Parse, substituting the @string replacements.
+                 ;; See Emacs bug#56475 discussion.
+                 (let ((bibtex-string-files `(,(buffer-file-name)))
+                       (bibtex-expand-strings t))
+                   (bibtex-parse-entry t)))
                 entries)))
     entries))