imap: fix custom FETCH commands to handle literal responses

Custom IMAP commands using -X (e.g. 'FETCH 123 BODY[1]') were only
returning the first line of responses containing literals, instead of
the full multi-line body data.

The issue was that custom commands route through imap_perform_list()
and imap_state_listsearch_resp(), which didn't detect or handle IMAP
literal syntax {size}.

This commit adds literal detection to imap_state_listsearch_resp():
- Detects literal syntax {size} in untagged responses
- Writes the response header line containing the literal marker
- Handles any literal body data already in the pingpong buffer
- Sets up transfer layer to read remaining literal data from socket
- Configures maxdownload and transfer size to include header + body
- Initializes pp->overflow to 0 when no buffered data present
- Modifies imap_done() to transition to FETCH_FINAL for custom
  commands that set up downloads

Test 841 and 3206 verify.

Fixes #18847
Reported-by: BohwaZ
Bug: https://github.com/curl/curl/issues/18847
Closes #19246

tests/data/test841

@@ -20,6 +20,11 @@ body
 </data>
 <datacheck>
 * 123 FETCH (BODY[1] {70}
+body
+
++ Curl did not used to like this line
+--
+  yours sincerely
 </datacheck>
 </reply>