The targeted UID EXPUNGE (IMAPFolder.expunge(Message[])) from #295/#318 throws
"UID EXPUNGE not supported" on a server without the UIDPLUS extension, which
broke delete/move entirely on rare self-hosted/legacy IMAP servers (#319).
expungeTargeted now probes UIDPLUS from the folder's own already-open protocol
(via IMAPFolder.doCommand, so it never opens a second connection + LOGIN and
keeps the one-connection-per-batch invariant of #125/#295). With UIDPLUS it
still uses the targeted UID EXPUNGE. Without it, it falls back to a plain,
untargeted EXPUNGE only when provably safe: the messages we just flagged are the
only \Deleted ones in the folder. When unrelated \Deleted mail is present a plain
EXPUNGE would destroy it, and there is no UIDPLUS-free way to expunge a single
UID, so we refuse and fail loud, preserving the #295 "never touch unrelated
\Deleted mail" invariant.
PII-free AppLog breadcrumbs record the fallback decision. Covered by GreenMail
unit tests for both branches (with UIDPLUS via the default probe; without via an
injected capability seam, since GreenMail always advertises UIDPLUS).
Closes#319
JMR-dev
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