[Sub-org] Fix Excel export hiding the row grouping gutter

[anishanne]

Summary of the problem

The collapsible tree added in #14540 renders correctly in Google Sheets but the grouping gutter is missing entirely in Excel.

Unzipping an exported file shows the row grouping is present and correct:

<sheetFormatPr defaultRowHeight="15" outlineLevelRow="3"/>
<row r="3" spans="1:5" outlineLevel="1">

...but so is this:

<sheetView tabSelected="1" showOutlineSymbols="0" workbookViewId="0"/>

showOutlineSymbols="0" tells Excel to hide the outline gutter. Google Sheets ignores the attribute and draws the grouping anyway, which is why it only reproduced in Excel.

It comes from outline_settings(1, 0, 0, 0). write_xlsx guards that flag with a bare truthiness check:

# write_xlsx-1.12.3 worksheet.rb:3618 — "Turn outlines off"
attributes << ["showOutlineSymbols", 0] if @outline_on

0 is truthy in Ruby, so any number turns it on — passing 1 for visible (meaning "show them") is exactly what hides them. The same bug on line 4322 is why exports also carried a stray applyStyles="1" despite auto_style being 0.

Describe your changes

• Pass false for visible and auto_style, the only value write_xlsx reads as off for those two. symbols_below/symbols_right use a correct == 0 check, so 0 stays right there.
• Comment the inversion so nobody "cleans up" false back to 1.
• Assert showOutlineSymbols is absent from the sheet in the existing tree spec, so this can't silently regress if the gem ever fixes its logic.

Verified against write_xlsx 1.12.3 directly — emitted XML after the change:

<sheetPr><outlinePr summaryBelow="0" summaryRight="0"/></sheetPr>
<sheetView tabSelected="1" workbookViewId="0"/>
<sheetFormatPr defaultRowHeight="15" outlineLevelRow="2"/>
<row r="3" spans="1:5" outlineLevel="1">

[garyhtou]

force merged bc GH actions is down; so merge queue doesn't work