Harden Visio comments loading to prevent XML DoS

[PrzemyslawKlys]

Motivation

• The new Visio comments loader previously materialized an unbounded visio/comments.xml part via XDocument.Load which could exhaust memory/CPU when fed attacker-controlled large or highly compressed XML.
• The goal is to prevent DoS by bounding decompressed part size, XML character document size, number of comment entries, and per-comment text length while preserving existing functionality.

Description

• Added size and parsing limits: MaxCommentsPartBytes, MaxCommentsXmlCharacters, MaxLoadedComments, and MaxCommentTextCharacters and a hardened XmlReaderSettings used for comments parsing.
• Replace direct XDocument.Load(stream) with LoadCommentsXml which reads through a BoundedReadStream and an XmlReader configured with the secure settings to enforce decompressed-byte and XML-character caps.
• Add runtime checks that cap the number of loaded CommentEntry elements and validate per-comment text length before constructing VisioComment objects, throwing InvalidDataException when limits are exceeded.
• Add unit tests to OfficeIMO.Tests/Visio.Comments.cs that exercise oversized comments XML, excessive comment entries, and oversized comment text, plus helpers that replace the visio/comments.xml part in a generated VSDX for the tests.

Testing

• Ran PATH=/tmp/dotnet:$PATH dotnet build OfficeIMO.Visio/OfficeIMO.Visio.csproj -f net8.0 and the project built successfully.
• Ran PATH=/tmp/dotnet:$PATH dotnet build OfficeIMO.Visio/OfficeIMO.Visio.csproj -f net10.0 --no-restore and -f netstandard2.0 --no-restore and both builds succeeded.
• Executed the Visio comment tests with PATH=/tmp/dotnet:$PATH dotnet test OfficeIMO.Tests/OfficeIMO.Tests.csproj -f net8.0 --filter FullyQualifiedName~VisioCommentTests and the filtered tests passed (Passed: 7).
• Also performed a dotnet build of the test project for net10.0 and it completed with no errors (warnings only).

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[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.33%. Comparing base (0d248cc) to head (7f4b605).
⚠️ Report is 154 commits behind head on master.

Files with missing lines	Patch %	Lines
OfficeIMO.Visio/VisioDocument.LoadCore.Comments.cs	68.18%	11 Missing and 3 partials ⚠️
OfficeIMO.Visio/VisioDocument.SaveCore.Comments.cs	75.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1927      +/-   ##
==========================================
+ Coverage   72.30%   73.33%   +1.03%     
==========================================
  Files        1956     2521     +565     
  Lines      225358   283242   +57884     
  Branches    48654    60594   +11940     
==========================================
+ Hits       162943   207716   +44773     
- Misses      40161    47886    +7725     
- Partials    22254    27640    +5386     

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