LaTeX reader: fix theorem label parsing (#9198)

Closes #8872.

In the current implementation , theorem labels in `theoremEnvironment` are
determined by the `LastLabel` in the current state.
This approach works well when the `\label{label_name}` is placed at the end
of the theorem body. However, when a label is placed at the beginning of the
theorem (another common practice) and additional labels follow in the theorem body,
`theoremEnvironment` incorrectly picks the last label (e.g., `\label{item2}` in #8872).

This patch addresses the issue by extracting the label extraction independently of the `LastLabel` state.

1 files changed, 45 insertions, 0 deletions

diff --git a/test/command/8872.md b/test/command/8872.md
new file mode 100644
index 000000000..fddcb3c47
--- /dev/null
+++ b/test/command/8872.md
@@ -0,0 +1,45 @@
+
+```
+% pandoc -f latex -t native
+\documentclass{amsart}
+\newtheorem{theorem}{Theorem}
+\begin{document}
+\begin{theorem}\label{thm}
+\begin{enumerate}
+\item \label{item1}text1
+\item \label{item2}text2
+\item text3
+\end{enumerate}
+\end{theorem}
+\end{document}
+^D
+[ Div
+    ( "thm" , [ "theorem" ] , [] )
+    [ Para
+        [ Strong [ Str "Theorem" , Space , Str "1" ]
+        , Str "."
+        , Space
+        , Space
+        , Emph []
+        ]
+    , OrderedList
+        ( 1 , DefaultStyle , DefaultDelim )
+        [ [ Para
+              [ Emph
+                  [ Span ( "item1" , [] , [ ( "label" , "item1" ) ] ) []
+                  , Str "text1"
+                  ]
+              ]
+          ]
+        , [ Para
+              [ Emph
+                  [ Span ( "item2" , [] , [ ( "label" , "item2" ) ] ) []
+                  , Str "text2"
+                  ]
+              ]
+          ]
+        , [ Para [ Emph [ Str "text3" ] ] ]
+        ]
+    ]
+]
+```