Export snippet matching without closing "@@"

[Tom Alexander]

I suspect this is a regression. I am using this test document:
```
@@latex:



\documentclass[margin,11pt]{res} % default is 10 pt



@@
```

and dumped the AST with M-x eval-expression (message "%s" (pp-to-string 
(org-element-parse-buffer)))

When using the latest in main (commit b89bc55867d7cb809c379d371d12d409db785154, 
Org mode version 9.7-pre (release_N/A-N/A-b89bc5 @ 
/usr/share/emacs/site-lisp/org/)) the "@@latex:" gets parsed as an export 
snippet despite not having a closing "@@" (because the "@@" is in a different 
paragraph).

When using the version of org-mode that shipped with emacs 29.1 (Org mode 
version 9.6.6 (release_9.6.6 @ /usr/share/emacs/29.1/lisp/org/)) the "@@latex:" 
gets parsed as plaintext.

The docs seem to show that a "@@" is required as the end of an export snippet 
based on the pattern "@@BACKEND:VALUE@@": 
https://orgmode.org/worg/org-syntax.html#Export_Snippets

AST from org-mode in main:
```
(org-data
 (:standard-properties
  [1 71 1 71 0 1 nil nil nil 3 71 org-data nil nil nil nil nil #<buffer 
*scratch*>]
  :path nil :CATEGORY nil)
 (section
  (:standard-properties
   [1 71 1 71 0 1 nil nil nil 1 71 first-section nil nil #0 nil nil #<buffer 
*scratch*>])
  (paragraph
   (:standard-properties
    [1 13 1 10 3 1 nil nil nil nil nil top-comment nil nil #1 nil nil #<buffer 
*scratch*>])
   (export-snippet
    (:standard-properties
     [1 9 nil nil 0 nil nil nil nil nil nil nil nil nil #2 nil nil #<buffer 
*scratch*>]
     :back-end "latex" :value nil))
   #("\n" 0 1
     (:parent #2)))
  (paragraph
   (:standard-properties
    [13 68 13 65 3 13 nil nil nil nil nil nil nil nil #1 nil nil #<buffer 
*scratch*>])
   (latex-fragment
    (:standard-properties
     [13 46 nil nil 1 nil nil nil nil nil nil nil nil nil #2 nil nil #<buffer 
*scratch*>]
     :value "\\documentclass[margin,11pt]{res}"))
   #("% default is 10 pt\n" 0 19
     (:parent #2)))
  (paragraph
   (:standard-properties
    [68 71 68 71 0 68 nil nil nil nil nil nil nil nil #1 nil nil #<buffer 
*scratch*>])
   #("@@\n" 0 3
     (:parent #2)))))
```

AST from org-mode in emacs 29.1:
```
(org-data
 (:begin 1 :contents-begin 1 :contents-end 71 :end 71 :robust-begin 3 
:robust-end 69 :post-blank 0 :post-affiliated 1 :path nil :mode org-data 
:CATEGORY nil :granularity nil)
 (section
  (:begin 1 :end 71 :contents-begin 1 :contents-end 71 :robust-begin 1 
:robust-end 69 :post-blank 0 :post-affiliated 1 :mode first-section 
:granularity nil :parent #0)
  (paragraph
   (:begin 1 :end 13 :contents-begin 1 :contents-end 10 :post-blank 3 
:post-affiliated 1 :mode top-comment :granularity nil :parent #1)
   #("@@latex:\n" 0 9
     (:parent #2)))
  (paragraph
   (:begin 13 :end 68 :contents-begin 13 :contents-end 65 :post-blank 3 
:post-affiliated 13 :mode nil :granularity nil :parent #1)
   (latex-fragment
    (:value "\\documentclass[margin,11pt]{res}" :begin 13 :end 46 :post-blank 1 
:parent #2))
   #("% default is 10 pt\n" 0 19
     (:parent #2)))
  (paragraph
   (:begin 68 :end 71 :contents-begin 68 :contents-end 71 :post-blank 0 
:post-affiliated 68 :mode nil :granularity nil :parent #1)
   #("@@\n" 0 3
     (:parent #2)))))
```

--
Tom Alexander