Fix XML 1.0 invalid control characters in DecodeJsonEscapedText

[Copilot]

Description:

DecodeJsonEscapedText() was decoding \b and \f JSON escape sequences into backspace (U+0008) and form-feed (U+000C) control characters, both forbidden in XML 1.0. This could produce invalid XML documentation — causing compiler warnings or errors in projects with warnings-as-errors.

Changes:

• \b/\f handling: Keep as literal two-character sequences instead of decoding to XML-invalid control characters.
• StripXmlInvalidChars(): Post-filter applied to the entire decoded result, removing any characters outside the XML 1.0 valid set (#x9 | #xA | #xD | [#x20–#xD7FF] | [#xE000–#xFFFD]). Catches invalid code points from \uXXXX sequences too.
• IsValidXml10Character(): Named helper extracted from the filter for readability.
• Tests: Two new unit tests — one covering \b/\f literal preservation, one verifying that \uXXXX sequences producing invalid XML code points (e.g. U+0007, U+001F) are stripped.

Example OpenAPI Specifications:

swagger: '2.0'
info:
  title: Response description with control chars
  version: v0.0.1
paths:
  '/orders':
    get:
      summary: Get orders
      responses:
        "400":
          description: "Bad\brequest\u0007"

Example generated Refit interface

public partial interface IOrders
{
    /// <exception cref="ApiException">
    ///   <list type="table">
    ///     <item>
    ///       <term>400</term>
    ///       <description>Bad\brequest</description>  <!-- \b kept as literal; U+0007 stripped -->
    ///     </item>
    ///   </list>
    /// </exception>
    [Get("/orders")]
    Task<IApiResponse> GetOrders();
}

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.47%. Comparing base (608eb08) to head (0d28979).
⚠️ Report is 253 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Refitter.Core/XmlDocumentationGenerator.cs	53.84%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #949      +/-   ##
==========================================
- Coverage   93.56%   93.47%   -0.10%     
==========================================
  Files          26       26              
  Lines        1632     1639       +7     
==========================================
+ Hits         1527     1532       +5     
+ Misses         37       36       -1     
- Partials       68       71       +3     

Flag	Coverage Δ
unittests	93.47% <53.84%> (-0.10%)	⬇️

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