Prevent quadratic DoS in chart number-format bracket parsing

[PrzemyslawKlys]

Motivation

• Chart number-format literals read from OpenXML can be attacker-controlled and the previous normalization logic called IndexOf(']') from every unmatched '[', producing O(n^2) CPU for long malicious inputs.
• The goal is to remove the repeated suffix scans while preserving existing behavior for matched bracket directives and literal output for unmatched brackets.

Description

• Replace per-'[' forward IndexOf scans with a single-pass tracker (nextClosingBracket) so the literal is walked once and closing brackets are advanced linearly; the change is in OfficeIMO.Drawing/OfficeChartDrawingRenderer.Text.cs in NormalizeDataLabelFormatLiteral.
• Add a regression unit test FlowDrawing_IgnoresBracketDirectivesInDataLabelNumberFormatsLinearly that verifies matched directives are stripped (e.g. "[Red]$0") and that very long unmatched '[' prefixes are preserved as literals without quadratic cost in OfficeIMO.Tests/Pdf/PdfDocumentChartDrawingTests.cs.
• No project target frameworks or packaging settings were changed and net472 / net8.0 / other existing targets were preserved.

Testing

• Repository static checks were run and no whitespace or obvious diff issues were reported.
• The new unit test was added but could not be executed in this environment because dotnet is not available; the test is included and should be run in CI or a local environment with the repository test matrix (for example: run dotnet test OfficeIMO.Tests/OfficeIMO.Tests.csproj for the relevant target frameworks).

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Codex Task

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.33%. Comparing base (20cf306) to head (9ec917b).
⚠️ Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
...ficeIMO.Drawing/OfficeChartDrawingRenderer.Text.cs	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1923      +/-   ##
==========================================
+ Coverage   73.29%   73.33%   +0.03%     
==========================================
  Files        2256     2521     +265     
  Lines      262814   283188   +20374     
  Branches    56792    60585    +3793     
==========================================
+ Hits       192633   207670   +15037     
- Misses      44317    47879    +3562     
- Partials    25864    27639    +1775     

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