fix: fill crash on a stray '}' before a template placeholder

[BigDataDZ]

Purpose of the pull request

Filling a template crashed with java.lang.StringIndexOutOfBoundsException whenever a template
cell contained a stray } before a {placeholder} (e.g. long prose or a JSON snippet in the
same cell as the placeholder — see #191, where the fill died with Range [1211, 1027)).

Fixes #191

What's changed?

In ExcelWriteFillExecutor#prepareData, the closing suffix of a placeholder was searched with
value.indexOf(FILL_SUFFIX, startIndex + 1). startIndex only tracks how far escaped
placeholders have been consumed, so the } search could start before the current {. A stray
} preceding the { was then treated as the placeholder's suffix, inverting the bounds of
value.substring(prefixIndex + 1, suffixIndex).

The suffix search now starts after the {:
value.indexOf(FILL_SUFFIX, Math.max(startIndex + 1, prefixIndex + 1)). A stray } is kept as
literal text (a}b{name} fills to a}bfilled); escape handling (\{, \}) and multi-placeholder
cells are unchanged.

Tests

New FillWithStraySuffixTest (round-trip): a template cell containing a}b{name} filled with
{name: "filled"} — crashes on current main, fills to a}bfilled with this change.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[skytin1004]

Thanks for the fix. I reproduced the failure and verified that this change fixes it. All tested cases passed. LGTM.