Rebase copilot/fix-xlsx-export-issue onto master

[Copilot]

Rebased the XLSX export fix branch onto latest master (6dd28dfc).

Changes

• Branch rebased from older base to current master HEAD
• Preserves XLSX export compatibility fixes:
  • Default font injection for empty reports
  • Worksheet relationship entries for empty sheets
  • Consistent r:id references across workbook files

Verification

• Build passes with no new errors
• No merge conflicts during rebase

Original prompt

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This section details on the original issue you should resolve

<issue_title>xlsx Export Issue again</issue_title>
<issue_description>If you open the exported xlsx with Excel 2013, it warns you that the file needs to be recovered.

In the contents of the xlsx file, I've noticed the following mistakes:

• \xl\styles.xml: no font is defined
• \xl\workbook.xml: r:id="rId1" is not correct
• \xl_rels\workbook.xml: there is no Relationship entry for sheet1, specified in \xl\workbook.xml

See the attached empty report.
test_report.xlsx
</issue_description>

Comments on the Issue (you are @copilot in this section)

@majorsilence If you would like you can send a pull request with a fix.

https://github.com/majorsilence/My-FyiReporting/blob/master/RdlEngine/Render/ExcelValet.cs and https://github.com/majorsilence/My-FyiReporting/blob/master/RdlEngine/Render/RenderExcel.cs are probably the files you want to look into.</comment_new>

Fixes #161

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