fix: parse LaTeX macros in multicolumn/multirow table cells (#3204)

* fix: parse LaTeX macros in multicolumn/multirow table cells

Table cells in multicolumn and multirow used raw LaTeX content
(e.g. \textbf{Header}) wrapped in LatexCharsNode, which passed
through _nodes_to_text() verbatim. Parse the content string with
LatexWalker so formatting macros get properly resolved to text.

Signed-off-by: majiayu000 <1835304752@qq.com>

* test: add regression test for LaTeX formatting in table cells

Add test_latex_table_formatting_in_cells to verify multicolumn/multirow
cells with \textbf, \textit, \tiny, and nested formatting produce clean
text output without raw LaTeX commands (issue #3199).

Also narrow except clause from bare Exception to LatexWalkerParseError
for both multicolumn and multirow parse fallbacks.

Signed-off-by: majiayu000 <1835304752@qq.com>

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Signed-off-by: majiayu000 <1835304752@qq.com>