feat(send): quote a long post's text in an expandable blockquote

A post whose text (the split `title` plus `content`, joined by
`site::compose_text`) reaches `CAPTION_QUOTE_TEXT_CHARS` — default 200,
`0` disables — now has that text wrapped in `<blockquote expandable>`
inside its caption, leaving the URL and author line outside the quote.

Applied at the send boundary (`send_media_sequence`, `send_animation` and
the inline answers), where the caption is already truncated and the same
cache snapshot supplies the text, so a fresh send, a link-cache resend
and a queued retry all decide identically. The text is located as what
follows the author link, with the visible prefix accepted as a match
because `truncate_caption` may cut inside it — that keeps the longest
posts, the ones that most need folding, quoted. Captions whose layout
moves the text elsewhere (pixiv's title-inside-a-link, a `/set_format`
that puts `{title}`/`{content}` first) stay unquoted rather than risking
a blockquote nested in a tag, and a caption that already carries one is
never wrapped again.

Telegram measures a caption *after entities parsing*, so the tags cost no
length and the 1024-character limit cannot be breached; retries replay
the unwrapped caption, so a threshold change takes effect immediately.
The edit-before-forward rewrite stays unquoted by design.

Verified against Telegram: a media-group caption built this way comes
back with `caption_entities` `url` @0, `text_link` @50,
`expandable_blockquote` @56 — the quote starts after the author line.

crates/xmedia-bot/src/media_sender.rs

@@ -327,9 +327,20 @@ pub(crate) mod test_support {
             &self,
             _chat_id: ChatId,
             _reply_to: MessageId,
-            _items: Vec<InputMedia>,
+            items: Vec<InputMedia>,
         ) -> BoxFuture<'_, Result<Vec<Message>, RequestError>> {
             Box::pin(async move {
+                // Record the captions exactly as Telegram receives them (only
+                // the first item of a group carries one), so tests can assert
+                // what a recipient sees.
+                self.captions
+                    .lock()
+                    .extend(items.iter().filter_map(|item| match item {
+                        InputMedia::Photo(photo) => photo.caption.clone(),
+                        InputMedia::Video(video) => video.caption.clone(),
+                        InputMedia::Animation(animation) => animation.caption.clone(),
+                        _ => None,
+                    }));
                 match self.next("send_media_group") {
                     Outcome::GroupOk => Ok(Vec::new()),
                     Outcome::GroupErr => Err(self.error()),