bug#73581: 29.4; Gnus: Error doing a search on nnmaildir with gnus-search-find-grep

i can confirm this on emacs 30.2 / gnus v5.13 with nnmaildir.

setup:
- gnus-secondary-select-methods: nnmaildir with (directory "~/Maildir/...")
- gnus-search-default-engines: (nnmaildir . gnus-search-find-grep)

gnus-search-find-grep fails: looks up nnmaildir-directory, nnmaildir
only has directory → "no directory found in definition of server"
(same as visuwesh msg #14 point 1).

now i use gnus-search-mu instead; also hit related bug#72196 (full group
names passed as maildir: paths). workaround: strip with
gnus-group-short-name before the mu command.

happy to test patches.